Someone else said it earlier better, but it boils down to "I can't play the DnD campaign I've played for over a decade without more house rules than pre-packaged rules". The 2E-3E transition wasn't seamless, but few major fundamental assumptions changed... And when they did, it was adding things, that I could chose to ignore or not. I didn't have to rewrite entire classes, re-create races, etc. Everything I needed was still there, there was just extra goodies that I could use or not as I saw fit.
I could point to other things, like flippantly changing things like core wheel cosmology or reinventing elves... And I don't mean re-inventing like "halflings lost weight", here. I could go into the death of part of my soul when I read the "agro rules". I could discuss the abomination that is the new monster manual, or how much I despise this or hate that, but really that's what it boils down to. 2E to 3E was a major change, but making the transition was mostly a matter of re-doing existing things with new rules. 4E doesn't have half the rules I would need to re-do an existing setting without massive, massive changes.
I guess if I wanted to sum up how I feel about 4E, is its like... a step backwards, it feels more like an RPG from a smaller publisher. It feels like the RPG that a company makes that has a system that works for it's setting, and a setting wrote around it's system, and that might be fine on it's own, and sure you can write your own setting for it's system, but the nature of it's system means that your setting is going to be similar to it's pre-assumed one.