First of all, Kaelik = win.
With that said the biggest thing is the blandification and the grind.
First, blandification. I play illusionists. I play shapeshifters. I play gishes. I play summoners. I play multithreats. I play necromancers.
4.0 does not fucking support any of this!
Instead, it tries to force four box design, except with smaller boxes. With fucking shit covered spikes on the inside.
Given that I've realized four box design is an utter fucking joke the entire time, why would I support something that tries to make me even more narrow and completely useless outside of my tiny little niche? I could play a mother fucking 3.5 Monk and get a better deal than that. And Monks have it worse than even Fighters.
Now the 4.0 fanboys will inevitably try to counter with 'just make it up'. Except there are several problems with this.
A: As Kaelik and others have already made clear, any character who has real fucking abilities automatically wins the entire game. Level 1 3.5 Wizard > Level 30 4.0 anything for actually doing shit that matters. Even the mother fucking 3.5 Fighter and Monk easily trump anything 4.0 has to offer.
B: The alternative is just more blandification and empty hand waving bullshit. Also, it quickly drains your wallet.
C: If I felt like designing my own entire system mechanics for entire concepts, I'd just make my own goddamn system and fuck spending my money on something that's useless to me because it can't stop blowing horses for pocket change to fucking do what I need it to do. As an added bonus, I could sell that system and actually make money instead of wasting it.
Next, the grind. A brief comparison between a Fighter and the high end of enemies across editions.
In 1st edition you did 1d8+6 or so, several times a round, at full THAC0. Meanwhile gods/dragons/elder demons had double digit HP, so this was seriously actually pretty fucking good. Of course those same enemies could 1-2 round you, but eh.
In 2nd edition, not so different. I forget the details, but now you were fighting stuff with around 140 HP on the outside instead of 88, and you had the same or better damage. You're still alright.
In 3rd edition, now 500-1,000 HP are the norm. Your damage is better than 1d8+6, but you now take attack penalties and can barely move while still being remotely relevant. Even so, with enough optimization you can plow through the big HP bag before it destroys you.
In 4th edition, now we're up to... what the fuck was it? 1,400 HP? Except the Fighter's damage is back where it was in 1st or 2nd edition... so he gets to have fun scratching the mob to death. Meanwhile it does the same to him, because a level 29 mob seriously cannot OHKO a level 1 character, much less do meaningful damage to a level 29 character. Then there's a fuckload of trivial minor bullshit like +1 for one round you're honestly expected to care about, and more to the point lots of shit that makes encounters even slower.
Seriously. 4.0 would have been better off as a computer game. Then it does the math for you, it tracks all the irrelevant bullshit for you, and because getting through a round means flipping through a few menus the fact it's taking some arbitrarily high number of rounds doesn't matter as much. Just like a FF game. Imagine how bad those would suck if you had to roll for 'Firaga' a bunch of times and you see why 4.0 Fails. As a pen and paper game is just grossly magnifies the preexisting Grind. Also, computer games do hack and slash better both because of the doing math for you, and because of the pretty graphics and such which further distracts you from the Grind. Meanwhile computer games do anything except hack and slash poorly, so the whole inflexibility, inability to affect the world, very crude social mechanics that only get implemented because of hardware limitations that prevent actual roleplaying? Perfectly at home in a computer game. A blatant insult to every pen and paper DM everywhere as it is directly stating that the DM's brain is incapable of handling making a real world that characters can actually influence, characters that aren't super narrow one trick ponies, and some social mechanics are worth a shit (even the Fail that is the 3.5 skill system is still far better than the Epic Fucking Fail Abortion that is 4.0 'Skill Challenges'). This is a 'Chrysler: Thank you America! America: Fuck you Chrysler!' moment. Except WotC instead of the car company.
This is just the beginning of 4.0 Fail. The tip of the iceberg, as it were. I could go on for several pages, but I'll just summarize by saying later versions are supposed to fucking improve upon their fucking predecessors, not pull a fucking Microsoft Vista.
I am the Crusader of Logic, and I approve this message.