What's interesting for those who say it's not D&D is - why not? What are the key things for you that are not present in 4e?
Vancian casting- should have been an option for at least one full casting class.
Spells that do more than hurt things or heal them.
Rules for DMs to make their own unique magic items- the Ritual only lets you make things that already exist, without explanation of how to extrapolate into things that do not.
True multiclassing that supports a wide variety of PC development styles. 4Ed's system would be a fine feat based supplement to the 3Ed rules, but its a poor replacement that would support only a mere handful of PCs I've run in the past 30 years- maybe as few as 25%.
CG, NG, LE, NE. The 9 point system is more realistic, flexible and robust than the 4Ed system. In all honesty, though, I'd have been just as satisfied if 4Ed had used a 3 point alignement system (G-U-E). The 5 point system is garbage, IMHO.
The other Planetouched races. It would have been easy as pie and
quite elegant to apply some of the mechanical solutions to introducing variety into the classes and made Tieflings one option of many under the umbrella term Planetouched (or Nephilim, or whatever).
FWIW, the same treatment listed above could have been done to Elves and/or Fey in general, saving valuable pages of the PHB for those of us who like (but don't drool over) Elves for some other info of use.
Missing Gnomes, Monks, Barbarians, etc., make it impossible for me to update my PCs and campaigns to the new edition without converting, adapting, or worse BUYING something. After playing for 30 years with several groups- one lasting 20 years and counting, one at 10 years and counting- I'm not really keen on retconning or worse, ditching everything and starting anew.
Negative racial stat mods so that critters look like they should. I'm sorry, but the MM's PC-ized version of the Minotaur is no more fearsome than a Dragonborn (a creature who is, on average, 50-100lbs and 9-11" shorter), and is certainly a lot smarter (on average) than most previous Ed's versions of the bully boys.
Inclusion of mechanics- healing surges, Ftr & Pal aggro, etc.- that are some of my least favorite features of arcade combat games and online RPGs.
A unified system for both PCs and NPCs. Now I have to learn 2 methods of generating personalities? FEH!
I'm all for giving novice DMs tools to work with (including copious examples), but at times, the 4Ed DM is treated like an idiot. Exhibit 1: the magic adjustment charts for monsters and NPCs. If I'm giving an Ogre a +3 weapon, I want it to give him a bonus of +3, not +2 as dictated by his adjustment. If I wanted him to have a bonus of +2, I'd have given him a +2 weapon.
And this is with only a week's worth of perusing the game.