Despise it. I want to find whoever thought it was aan improvement and beat them severely.
The characters are now nothing more than a group of index cards with almost no room for roleplaying. Your actions are severely limited by the way that “encounter” and “daily” powers work. Sorry, but if I want to cast a certain spell more than once per encounter then I am fucked unless I take some other odd class power. Or maybe one from a paragon path.
The character creation/advancement is totally fucked up. You need at least three books to get a list of half the stuff your class can do and even then you have to make too many choices for it to be easy. In older editions, it was recommended that new players avoid taking spellcasting classes since the spell lists were too confusing for someone unfamiliar with the system. Well, in 4E, all characters are nothing but a spell list!
Furthermore, the power curve goes from bleh at low-levels to beyond god-like in the mid-20s. Put another way, a 25th-level 4E character has more powers than a 100th-level character from earlier editions.
Sure, Shadowrun made some drastic changes in 4E, which is pretty much a totally new rules set from 1E-3E, but at least they didn’t totally fuck it up like D&D did. And GURPS 4E is basically 3E with a few simplifications and major changes, but quite obviously an evolution rather than a re-do since the basics are the same as they’ve been since 1986.