My view of the good, some of which has already been mentioned, in no particular order:
1. Innovation in mechanics. MMOs have been borrowing from pen & paper for a long time, it was nice to see DoT, HoT, and explicit role mechanics feed into the tabletop setting. While I don't think they worked well (they proved to be fairly clunky), i think it was a solid attempt and a good thing to try. It also showed that some things, e.g. a proliferation of short-term buff/debuff and damage-over-time mechanics are tough to track on paper. However, experimentation in a high profile game (thus lots of people trying it out) was worthy of a try.
2. A determined dedication to a fixed technical vocabulary. By being explicitly technical, they removed a lot of the squishiness of earlier editions, and gave a broad range of pre-defined effects to work with. This made matching rules up with other rules, or working out outcomes, to be straightforward.
3. Scalable monsters, and a tool to scale them up/down. Not only do you get a wide variety of levels in the monster manuals, the mechanics are transparent enough that you can scale them outside of the suggested range fairly easily (and some, e.g. dragons, cover the whole range of levels).
4. Distinct, easy, self-contained monster stat blocks. This made referencing during the session easier, especially given that my group rarely met more than once a month (so details were easily forgotten).
5. Minions! (Yeah, I know, people would use masses of orcs under a wizard or whatever to challenge level 7-9 parties, but it's not the same thing. An explicit cannon fodder rule rocks. Actually, I think GURPS Martial Arts had "mooks" rules, and maybe HKAT did as well...) And, in the same vein...
6. Real Bosses ("solo" encounters) with very challenging stat blocks (hundreds of hit points) and group-countering mechanics. This could result in a truly epic feeling fight in a way that never quite felt the same in either 2e or 3.x to me.
7. The cosmology is simple but adaptable. I was a little leery at first, but then realized it had usable detail (e.g. Gloomwrought's specifics) while staying flexible enough to be adaptable to many worlds or systems. I slotted it in comfortably with Mystara with minimal difficulty, and it felt "right".
8. Balanced mechanics: Now, this may be controversial, but I appreciated that, within a given role, the classes were pretty darn equal. That meant two things: (1) jealousy between damage-dealers was less, and (2) super-optimization was tougher. The latter meant that if someone was saying "I know 1st level rogues get one bonus die, but my multi-class build lets me get three", the player was probably mis-reading the rules. (Side note: in the specific case, they ignored the "...which only applies to the base class abilities" rule which was explicitly in the rules.) In effect, it gave a "sniff test" of interpretations -- if it seemed wildly out of whack, someone was probably misreading something. I'm still of two minds on the roles, and if we do a "Nice try, but missed" list, I'd have a lot more to say. For my specific group and experiences, though, the balance within the roles was a good thing and worked well for us.
9. Explicit progression path: It was nice to have the treasure and power level expectations laid out so handily. Yeah, the parcel system took some of the fun out of looting and dungeon design, but it did give very clear guidance on where the designers expected the PCs power levels to be. In earlier editions, this was spottier -- you pretty much had to guess, have experience at what you were comfortable with, or try to extract guidelines from the published modules. Most people agreed that a +1 weapon was a good reward for levels 1-3, but does the +2 come in at 3, 5, or 7? Is +5 expected at 10, 15, or 20? I liked having the suggested guidelines, but never considered them strict barriers.
10. I liked the really fantastic versions of the races. Maybe I've spent too much time in WoW (and with Warcraft's art), but the half-elemental version of the Genasi, or the obviously celestial Deva or fiendish Tieflings sat well with me. I know, it was a significant change in the art direction, but it tickled my fancy after almost 30 years of beautiful but fairly staid art.
11. I liked Perception being both passive and combined (spot + listen). One of the challenges has always been being fair to the players -- if I roll their checks in secret, it feels like I've taken their control of their own fate away, but if I have them roll, it's really tough to keep the information from going "meta" ("Oh, he rolled a spot check? I'll have my character roll one, too!"). I usually worked around this by a sheet of pre-rolls, rolled by the player but used by me (in order) as required. This mechanic works better, and I'll probably consider keeping it in other editions. That, and and the skill simplification of just "perception" covering all the bases.
There were other things I liked about the edition, but those are the highlights. Some of it is purely taste, some of it is a shift in my view on design and balance, some of it is driven by my particular players.
My "nice try, but it missed" list is probably just as long, if not longer. My list of flat-out fails is very, very short.
I like 4e, a lot. It might even be my favorite edition since 2e, which also took me a little while to make my peace with. If they weren't orphaning it, I'd probably continue playing it until my campaign ended.
...although orphaning it may open the door to me tweaking it in a way I couldn't while they were actively developing the system. Hmmm... That deserves some thought...
 As a riff on this -- when I played Ultima VII, I wanted a similarly easy-to-understand tabletop mechanic, which was present in Dangerous Journeys Prime. That may have driven my initial interest in that system -- and I've long felt that the speed of computer play and game iteration makes it the better laboratory for mechanics experimentation. You can do a year's worth of tabletop gaming in a night.
 Speaking of MMOs and tabletops cross-feeding, WoW's latest expansion (Mists of Pandaria) have packs of same-level, lower-power creatures which are fairly reminiscent of the minion mechanic.
 Sometimes I like things being mixed up or changed just to reexamine how it was or my own expectations. I adored Spelljammer and Dark Sun for mixing it up so much. So, the 4e direction felt novel enough to be intriguing and thought-provoking -- what can I do with it? Does it fit in? Can I work with it, or do I need to tone it down? I decided to tone it down for Mystara, to fit with how existing materials worked, but the option was there to keep it fairly crazy.
(maps, notes, merchant houses)
NaNoWriMo: Winner 2013, 2014, 2015; Camp NaNoWriMo: 2014 (April, July), 2015 (April, July)