Actually, they're both 'open' world hack-and-slash RPGs with a poor storyline, general gameplay (both have some interesting facets, but they aren't anything spectacular) and could be better in many ways.
Yes, I'm saying Diablo is meh. *Runs like hell from the internet*

Are we talking Diablo 1 AND 2 or just Diablo 1?

Your opinion of Diablo being 'meh' being one thing (which I sort of agree with *ducks*) - Diablo 2 isn't an 'open' world game (either in the limited sense, or the broad sense of the term). You are on rails following the quest line with some basic side quests here and there. From what I understand Two Worlds is more akin to the Elder Scrolls games in regards to open world.
Open World implies that you have a freedom to go anywhere at any time and basically ignore any 'main' quest lines there are - You can't ignore the main quest in Diablo because you need to advance it in order to move on to the next big area. Open World also implies that there are few to no artificial borders stopping you from visiting different locations - even if those locations contain monsters that one-hit kill you, Diablo has no such freedom (from what I have read on Two Worlds it does have that freedom going for it).

(Couldn't be assed to mess with the quote system, so this is in reply to both Hammerfall and carlosjuero.)
Yes, I'm talking about both Diablos. Neither are particularly interesting for me, at time of launch or now. They just aren't the godlike games everyone I meet seems to think, IMO.
As for open world, no, Diablo isn't. Open isn't the right word, or the right way to compare the two.
What I basically meant was, as said before: They're both mediocre hack-and-slash RPGs that did a few interesting things, and don't (anymore) have any major flaws in their systems. They are both good examples of middling games in both scope and depth. Neither are particularly good, but they can both be quite fun with friends/in the right circumstances.

And here we go again with the unexplained opinions - you cannot impose your own standards and tastes on other people. Having said that I've only played the first Diablo (so far), I had a big lot more fun with Two Worlds. Why shouldn't they be compared? They're in exactly the same category, with slight differences in focus. They're action-based RPGs that do not offer much in terms of characters, story, or quests, but live from their mechanics, fights, exploration, and character levelling/equipment. Two World is Diablo in 3D, with less focus on dungeons and more focus on freedom/exploration.
I played Diablo through once. If you're done, the only thing left to do is start over if you are fond of the mechanics and exploring the dungeons. The same is true for Two Worlds. Personally, I have much more interest in replaying 2W though because I enjoy the world much more (even just exploring it is fun), and it is much more complex. By all means you do not have to agree, but putting down a game without giving any reasons at all is not very constructive in my opinion.
People are so quick to defend the successful mainstream games like Diablo. That it is completely bland content-wise is okay because that is not the game's main focus - and I agree, it is okay for the game to put its focus elsewhere, and it still makes for a very good game. Yet when talking about Two Worlds, suddenly there is no excuse anymore for it having its own focus and not excelling in other aspects such as quests and storytelling? That's called a double standard.
Give me one element in which Two Worlds cannot compete with (the original) Diablo?
(This of course requires one to have played both Diablo and Two Worlds, the latter of which I understand many of those giving unfunded negative reviews did not bother to do).

Please don't even joke about Diablo being an RPG, action or not.
It's a freaking dungeon crawler at best. Borderlands is more RPG than Diablo.

I don't know if I really like open-world adventure games in general. I don't think I've really played that many of them. Morrowind is one of the most popular, but while I load it up now and then and have fun for a while, it tends to get repetitive for me by level 12 or so. Maybe I'm doing it wrong?
On the other hand, I absolutely adore the Might and Magic series - I think I get the same sort of joy out of exploring the M&M games (particularly the newer ones, oddly enough, those having more expansive and detailed worlds) that some people get from the Elder Scrolls games, though maybe for different reasons. They're not as freeform or elaborate as Morrowind - you can't arbitrarily steal somebody's gold plates and sell them in the next town over, for example - but I do feel largely free to take on the world however I see fit, which to me is what's important.
I did a double take. I read that as "Battletoads."

While you can compare the core gameplay of Diablo to Two worlds, i'd disagree with the comparison as a whole.
Diablo 2 had the "it" factor. It was an online focused game so the story shouldn't be compared to Two Worlds. Diablo 2 had great (imo the best) skill trees, multiple classes, not only tons of loot, but loot you WANTED to go out of your way to get and a great community thanks to Battle.net and the game's co-op and competitive nature. Plus Diablo 2 had all the atmosphere you could ask for. It pretty much did everything it focused on, and did it better then any company even tried to come close to. It's no wonder video gamers refer to most action games as "Diablo clones." And usually those said Diablo clones are complete failures because nobody really knows how to give games the said "It" factor.
Trust me, i'm not the one to completely defend high profile games neither am i a Blizzard fanboy (which I might sound like in this post)... My favorite games are usually lower profile games (Gothic 1/2, Arcanum, Divine Divinity, Drakensang, etc)..
Now whether Diablo 2 is even an RPG (I believe it is) or not is a whole other discussion. Nobody can deny it has some great character development/customization though. We'll leave this all to that Bioware guy to decide though. :P :P

We'll just have to agree to disagree then.
I think your view of Diablo 2 (as many other peoples is) is skewed by. . .something. Maybe nostalgia, maybe something I never managed to find when I played through them.
It just isn't that good of a game to me. Ah well, life rolls on. Bring on the next Elder Scrolls.

I'm playing Two Worlds now and will go ahead and join the 'love' club, but there's something I don't understand. Are drarwes some kind of general evil around there, or is there any other reason they attacked me first time I have encountered them?

I haven't reviewed Two Worlds, because I don't think I've played it enough to genuinely give it a fair shake. I also wasn't attempting to impose anything. I'm sure there are some people out there who genuinely love Two Worlds, and I trust that by saying that I don't enjoy it much, it won't affect their own enjoyment of the title one jot. (frankly, if it does, you might want to consider that perhaps you weren't really enjoying it that much in the first place).
As to why the two shouldn't be compared, it's because I think that Two Worlds can only look worse for the comparison. Technically, Two Worlds is leaps and bounds ahead of Diablo.
Diablo is hard to fault, though. It's a fantastically well refined game design. The levelling curve, the introduction of new elements, the careful balance of recieved loot. Even thoughtful elements like the static "hub" section and dungeon levels randomly generated with every new playthrough... Diablo is a certified genre classic for a reason, and it's addictive qualities were honed to a razor edge. It would not surprise me in the slightest to find that World of Warcraft used exactly the same loot tables as it's older sibling.
It may also interest you to know that Diablo did actually have a rather deep and engrossing back-story (much of which was covered in the surprisingly weighty manual) rather than directly in the game. A lot of people tended to click through the words though, and at heart, Diablo is a roguelike which means it has a plot you can summarise as "reach the bottom of the dungeon and kill the capital D".
To summarise, Diablo is one of very few games I have gone from playing the demo of at 6pm one evening to owning a full retail copy of the game by mid-day the next day. It being a successful game has nothing to do with that (though it being a successful game might well be attributed to the fact that it's a bloody good game).

Diablo is basically a graphical roguelike, the scion of a million text-based RPGs, stripped down (only a little) to be a lean, mean, action-based looting machine.
As such, I believe that it's undeniably an RPG (though as you say, it leans heavily towards the exploration and dungeon-crawl elements - that's fine, though, many games take different parts of the D&D experience and do their own thing with them).

I understand. I was just reacting to the vibe some people here created, blaming those who wrote positive reviews (and there's quite a number, here and on bigger review sites) of maliciously misleading prospective buyers. The truth is that I really loved the game, in my list personal all-time favourite action-based RPGs it comes right after Gothic and Morrowind. It's just that some people claimed that nobody could possibly like Two Worlds and anyone who claimed otherwise was lying and misleading people, for some reason or other. I completely see the points of criticism in Two Worlds, and it definitely lacks a lot of the polish that other games, especially bigger budget titles, sport. It didn't detract from what I personally enjoy the game for though, but I also understand if people with other tastes find that it makes for a much worse game.
As for Diablo, it does have that incredibly addictive quality to it simply because of extremely well balanced gameplay, which even infected me as I have played it a lot longer than I thought. The truth is though that Two Worlds evokes a very similar addictiveness in me, although reading Hammerfall's post I agree that the two games are really more different than it might seem superficially.
And as for the pacing of Two Worlds - I think open-world games need a lot less of that anyway. Freedom means that you can also over- and underestimate yourself. Morrowind didn't have much pacing either for that matter, it forced you to find your own balance.

May as well throw out another reason to get Two Worlds. There is a SDK available for modding.
One disappointment I have with Two Worlds is that the amount of backlash it got scared a lot of modding groups away. I look at it like this: It's may be flawed to some, but the tools are there to change it. If you're a modder, why not give this title a look? I know I give every title that has modding tools available a look, because I love modding. I also love commas.
I don't think the modding potential has really been exploited. This and The Witcher. It's a shame, because it discourages devs/publishers from bothering to put together a public SDK.
Ok, I'm a little biaised in that I enjoy this title quite a bit, but I'd love to see modders create for this game a litle more. It's not broken like everyone is saying and any perceived problem can be overcome.

Why is a game that's less than 3 years old, saw release on current-gen consoles, and whose sequel is right around the corner appearing on GoG? This one seems a little too new, even with some of the other releases that have been made here.
edit: er, i should add not that I'm sad about this. I've already bought it :) I had it for 360 and have wanted to get it on pc for a while. I'm just confused as to why a game so new that it's still on most store shelves would be on "good old games"
edit2: wow, reading comments in this thread, I am glad I waited until the GoG release to get the PC version. The DRM sounds nasty in this one!