I... don't get it. What does it mean "compatible with D&D"? How would the two connect? How would I... what? I don't understand at all. Is that just a D&D rulebook with names changed to fit Middle-Earth? And how would that work? D&D style magic wouldn't fit at all. Is it supposed to be "compatible" with the monster manual? Most of those don't belong in Middle-Earth at all, unless you go all Sandworms of Erebor, Peter Jackson style and say "fuck it". But in that case... I can just take the D&D rulebook and run the game in any fantasy world I want, can't I?

I must be missing something.

D&D is (once again) an open rules system. So it's a Middle Earth setting with D&D rules.

I get that. I don't get the "compatible" part. Compatible with what exactly?

With the basic rulebooks.
(and I guess with any supplements you want)

My understanding about D&D editions is this:

1e and 2e are fairly similar to each other, to the point where one could take rules from one edition and use them in the other with only minor adjustments. 2e did make some improvements, such as making THAC0 improvements more regular (you no longer have to memorize a table), relaxing the stat/level restrictions on demi-humans, and getting rid of the sexist limit on female strength. Some of the classes were removed in 2e (Assassin and Monk), and some were changed (Rangers no longer get mage spells, Bard no longer has complicated dual-classing requirements).

3e changed things significantly. Stats no longer need to be extreme to give a modifier (in 1e/2e, sometimes there's no meaningful difference between 6 and 14 in a stat), skills and feats were added, multi-classing was changed drastically (in a way that makes spellcasting multi-classes significantly less useful and made dipping into a class for 1 level possible), and the rules were re-worked to get rid of "lower is better" aspects. One change I don't like is the dependence on positioning: the Attack of Opportunity and Sneak Attack mechanics require keeping track of positioning and are not easily removed because too many rules (including some feats that are prerequisites for other feats) depend on them.

3.5e is fairly similar to 3e, making some changes to the rules, some good, some bad.

4e, from what I understand, is a completely different game; IMO it should have been called something other than D&D. The whole system of powers, in particular, is completely different from how earlier editions worked.

5e, from what I understand, is more like pre-4th edition, but of course has had some revision. Some of the mechanics are derived from 4e, but the core of the game is more like older editions. In particular, you are back to something similar to the old magic system. (Not exactly, of course; all spellcasting is spontaneous with the option of changing your spell list when you rest.)

But... how? I just can't see that working. I mean there are plenty d20 systems out there, but they are not compatible with D&D. Can I be a Tiefling in Middle-Earth now? I'm sorry, I'm probably being terribly dense right now, but I just don get how that's supposed to work. I see it like saying that there will be a Star Wars d20 game "compatible with D&D".

Well, there was... Star Wars RPG by WotC d20 system.
It's basically the same thing here. You're arguing terminology. They don't mean that you'll have a tarasque in Middle Earth. (but you could if you wanted to, and now I kinda want to)

Ok, so basically I just totally misunderstood what they mean by "compatible with D&D". It's not actually compatible with D&D itself, as in "you can combine the two", it's just a d20 system adapted for Lord of the Rings. Ok, that makes more sense.

Exactly. But I guess you could also combine the two, if you were so inclined. And it's not the d20 system anymore, by the way.

Knights Of the Old Republic games had a very similar setting to D&D 3.0 rules.

The chinaman is not the issue here, Dude.

Are you talking about drawing a line in the sand, Dude?

It's soooo much simpler than 5th edition Shadowrun. I don't know how similar Shadowrun 4 is to 5, but in five there's nearly 30 skills, and nine attributes. There's an action for more things than you can shake a stick at.
I get a different feeling between the two. In shadowrun it seemed like there was a specific rule for everything, and you had to look it up each time. In D&D it feels more like they've given you a framework that fits easily whatever you're trying to do. There's enough skills that you're going to have a relevant one %90 of the time. In Shadowrun there were four different engineering skills! More realistic, yes, more fun, no.
Don't get me wrong, the Shadowrun setting is amazing. HBS's game capture it so well.
If/when I play Shadowrun again, I would probably do quite a bit of modding to the rules system and chuck a bunch of stuff out. In D&D there's no need to do that.
... I just realized you weren't really comparing D&D and Shadowrun, but the two version's of Shadowrun. Maybe someone else will find this useful.

I think it does mean you can combine the two. Not that you have to, just that the option is there. So that, for instance, you could use an NPC from another campaign/adventure and bring them into the LotR world. My $0.02

There was an RPG based on Middle Earth called MERP I seem to remember.
M.iddle
E.arth
R.ole
P.laying
Think it was released by Iron Crown? Although this may be way off course!

I believe I actually have this one somewhere. I believe it actually is based off some other system, but with a lower level cap.

I do remember it having open-ended rolls, a mechanic I don't remember being in any version of D&D.