http://www.youtube.com/watch?v=NqiulSLFncc
Just lay your eyes on this sick puppy. Can you hardly believe that it's built using StarCraft II's Map Editor?

Very nifty, but I am always hesitant of any game whose appeal is going to be based on user-content (admittedly, SC2 has an SP campaign and Koeran-friendly MP :p). Especially in this day and age.
Take a look at UT3 (and, to a lesser extent, 2k4). Those flopped because the core game, while interesting, was largely expected to be overshadows by mods (and why not? look at UT and Quake 3). But the problem is that expectations for mods have gone up, so most people can't actually make them. And the people who CAN make mods don't want to make simple gameplay tweaks and the like: They want to do total conversions (because everyone wants to be the next Counterstrike or, to a MUCH lesser extent, the next Red Orchestra).
Already starting to see this with SC2. There is the FPS/TPS gimmick, which is great (Blizzard felt it great enough to showcase it). But it is never going to replace a proper FPS or TPS, and will stay as a gimmick.
So that leaves us with the RTS-esque mods and maps: We probably won't see too many singleplayer campaigns of worth (the original SC STILL doesn't have them, so it is unlikely we'll see many good ones for SC2).
And as far as MP goes: While I admit that my SC experience is less than most, most maps seemed to either be BGH for gameplay or User-Map-Settings (and of those, they were all pretty much Pokemon, tower defense, or DotA). And honestly, I don't think that is likely to change.
Don't get me wrong, I am sure that SC2 will stand on its own merits. Apparently the SP campaign is pretty good, and it is Starcraft: People are going to play the multiplayer.
But the Map Editor, as much effort as they put into iit, is probably going to remain in the same spot as the SDK and map editor for pretty much everything that has come out in the past few years (except maybe NWN/NWN2 and Dragon Age, which were mostly used for mods to improve the core game): Useful for gimmicks, and that is about it.

The original Starcraft's appeal (for me) was user-made content. It more than exceeded my expectations. I see no reason why Starcraft 2 won't do the same, as it has a much, much stronger map editor.
Bounds, RPGs, Snipers, Madnesses, TD, Bomberman, and maps that created entirely new genres were just a fraction of the maps SC had to offer. I'm confident mappers in SC2 can do the same.

Was this made using the map editor alone, or is it a mod?

This is even more impressive:
SC2 Final Fantasy
Hope someone makes an RPG out of it.

Map editor alone.

Damn, that's some impressive workmanship then. I was going to buy StarCraft 2 at some point, but now I feel like I need it right now just for the map editor.

That's the primary reason I bought it.

So what kind of maps have you made?

None yet. I haven't had much time due to work, family, and Minecraft. I've mainly just played around with the editor and tried to learn most of what it can do.
In the original Starcraft I made bound maps though. Not sure what I will do in the SC2 editor yet.

The modding tools are just like topping the crack/marihuana ball with a fine layer of cocaine dust for that extra bit of ... flavor.
Blizzard sure know how to make a product you'll be obsessing over for years.

1) What on Earth gives you the idea that UT2004 flopped in any way?
2) While UT3 was certainly not as successful as was expected, the reasons are varied, and expectations of mods over core game is far down on the list, if it is there at all. Major contributors to the game's failure are:
a) At the time of release, the game was an obvious console port. This is a bad thing to be for the next installment in a historically PC centric multiplayer FPS franchise.
b) At the time of release (and for several months afterward) there was no Linux server for the game. The core audience they were hoping to entice into buying the game (the large UT2004 fanbase) and to run servers for it, were all running Linux servers. From a marketing standpoint, the low priority on making a Linux server available to the very people who needed it to provide Epic with a market for their game (by hosting servers for it), must be said to be catastrophically stupid.
c) The singleplayer part of the game veered wildly away from the established formula of the franchise. The game is called Unreal Tournament 3, and they removed the tournament part from the game. They kept the gameplay, but shoehorned it into a far fetched, illogical plotline, revolving around a war between humanity and an alien race. This led to some ridiculous explanations of how the gameplay fit into it all, e.g. the Field Lattice Generator, or FLaG. This may have been an attempt at humor, but was not well received.

I suppose I am mostly meaning mod-wise in the case of 2k4 (hence, the lesser extent). Everyone expected mods out the whazoo (and why not? look at Quake 3 and the good UT :p), but didn't even get a wide variety of mutators.
I mostly mention that since it was the tipping point. After that, mods (as we once knew them) largely died down, to be replaced by countless half-finished total conversions.
Also: I forgot to list the TES games and Fallout 3 as games where modding is still very much alive (although, there are arguments to be made that it is mostly just two or three real "mods" and a bunch of retexes so that you can walk around the wasteland/cyrodil with nipple clamps).

Well, all I can say is that I disagree. I have seen so many brilliant mods for UT2004. All sorts of content, from new skins to existing models, whole new models, new weapons, game types, mutators, maps, and of course total conversions. Basically, UT2004 is one of the most modded games ever.