When 3e came out, I thought it was exactly what I wanted. It was very similar to my own attempts to revise D&D. I adopted it whole heartedly. I argued with naysayers on rec.games.frp.dnd as to why they should give it a try.
But I became increasingly frustrated with it. I won't go into any of the wherefores. I don't even know if my analysis of why is accurate. All I know is that it was frustrating for me.
On the other hand, over the years I've come to appreciate the logic behind classic D&D and AD&D. I've found it isn't as illogical, inconsistant, or unrealistic as I used to accuse it of being.
So, now, I find the 1981 Basic/Expert (B/X) sets to be not only my favorite edition of D&D, but quite possibly may favorite roleplaying game. (Classic Traveller scores highly as well.) I'm working on a new campaign world & running occasional one-offs until I get a chance to start a full blown campaign. I'm enjoying it all so much more than I was enjoying 3e.
Now, that's all speaking from the DM PoV. I enjoy playing 3e. (Although, I might enjoy other games a bit more.) I'm currently playing in a 3.5 campaign that I'm enjoying a lot. (Incidentally, it
might have been a OAD&D campaign, but I think I put the kibosh on that when I threatened that, if we played OAD&D, I'd insist we use the weapon v. armor adjustments.

)
I use a homebrew world with B/X D&D. I do not think classic D&D was created for Mystara. Rather, I'd say it was the other way around. OD&D was born from both Arneson's Blackmoor & Gygax's Greyhawk. B/X was founded heavily on OD&D, but was clearly, I think, meant for homebrew worlds. The many OD&D campaigns around Lake Geneva then were each set in homebrew worlds rather than Greyhawk or Blackmoor.
(Well, they were all set in worlds that were substantially similar. "World" may not be the best word, though, as they were sometimes seen as different regions of the same world. PCs could travel from Greyhawk to Blackmoor to Lendore to Aquaria to Kalibruhn. Plus, they shared a lot of aspects: Usually the same races & monsters & spells &c.)
What became Mystara had its birth in the 1981 Expert set. It was clearly, as I read it, merely an example and not considered the canonical setting it evolved into as they continued to locate modules in it.
I do think it is sad that--since classic D&D, AD&D, and 3e are really different games--that they shouldn't all be in print as separate products instead of being considered different editions of the same product.