Despite playing 4E (and occasionally liking it) I'm pretty much disappointed with some of its core design tenets. I'm doing my best to counter them in my homebrew 4E, but in doing so I need to rely on previous D&D material.
1. High level monsters have been nerfed to a degree that they utterly destroy dispense of disbelief. At level 35, Graz'zt can teleport for a total of 1 square more than a first level eladrin. Which is 30 foot total. If you recall, in previous editions Graz'zt could hop dimensions in the midst of a fight. Well, here 4E designers decided that they didn't want DMs to handle such heavy handed decisions by themselves - the ruleset takes care of that now. Personally I prefer a ruleset that's designed for self-respecting DMs.
2. The new planes are basically "more of the same" that is the new core material world. WotC eradicated any source of extraplanar danger - from the acid-covered inside of a Dreadnought's stomach (pretty tough if you get swallowed by it) to the power of the underworld river Styx to cause total amnesia to you character if you fail a save roll - and replaced it by a plane's "psychic signature". That's a purely emotional impact on the PCs which (the 4E designers go out of their way or asserting) has NO mechanical impact on the PCs. Imagine if the PCs could experience something truly dangerous, disruptive to their XP accumulation - UN THINK ABLE.
3. WotC designers went too far in their attempt to eradicate "black box" thought processes on the DM's part, where such processes include (for instance) the DM calculating a spell effect where only the final element in a long, long chain of reasoning impacts the game (the spell's damage).
Look at the new 4E Drow module P2 ("Demonqueen's Enclave"), and you'll see a monster ("Shunned") that has an encounter power which is labeled "Spider Surge". According to the power's definition, that power enables the monster to cause a square on the battle field to inflict poison damage on any PC who enters that square (the monster has limited control as to where that square is, i.e. can move the square around).
Compare this to that monster's description in 3E Fiend Folio/Drow of the Underdark: it "can spit spider swarms onto its enemies - goto Monster Manual page 229 for spider swarms". Ok, so I had to look up another book to see how the thing worked, which is clumsy at the game table (and reason number 1 why to shorten such "black box" thought processes on the DM's part - looking up one thing, doing a calculation, looking up another book, and then, a minute later, informing the player of the actual impact on his PC). But at least I knew HOW the monster's attack worked out in-game. As a DM I had a chance to relate the effect in purely narrative terms to my players. If I DM 4E, I can only say to my players "ok, my monster is doing a Spider Surge - watch out, that's 1d6 poisonous damage on you, Ralph". THIS is where I think 4E kills immersion. Its power descriptions - whether the PCs or monsters - are like a Magic the Gathering card: the flavour is just not enough
to translate into actual gameplay. In response, 4E supporters tell me "that's for you to make up". Well, guess what, with 1000 nearly identical mechanical effects, it's become impossible to decipher a spell's effect from its mechanics and/or name
so that coming up with my own fluff is a futile exercise on random guess work based on nil base material to inspire me. Previously, we had sleep spells, polymorph, etc. whose descriptions and/or names helped you to translate their effects to the game in narrative terms
because they didn't have miles of distance to cover between their narrative and mechanical dimension (<--that Mearls' design constantly creates such distances is his most considerable design weakness in my opinion). Now (in 4E) we got "Riposte Strike" and "Wolf Pack". They are cool names, but hollow. They lack descriptive power. And that, my friends, kills roleplaying at the micro level. Since roleplaying D&D, for me at least, always emerged at the micro level (e.g. how to disarm a trap), that pretty much kills roleplaying in D&D for me.
PS. THe following inspired me to write the final bit.
So what does 4e do? They decide to keep the idea of having complex rules to fiddle with, but throw away the essential piece that made the game a "role-playing game"... the rules actually reflected what was going on in the world! Now instead of having rules to help a DM through a dilemma (like, handling a wrestling match on a greased tightrope), we have rules just for sake of having them. Alright, so playing with the game's rules is fun I guess. Um... yeah.
Wait, I can't even pretend anymore. Having rules that don't represent anything in the game world is TOTALLY USELESS... because that's what the game rules are supposed to do in the first place... adjudicate elements of chance in the game world! Cripes! If I liked rules because they were interesting to tinker with, I'd play a game without a tremendous amount of overhead in time and money... like Monopoly or something.