I should start off by saying I've never played in a system that I straight up didn't like, and I prefer to focus on the positive and constructive criticism.
My friends and I tried 4e about five years ago and finished a whole campaign in it up to around Level 10. (Side note, it was a pretty cool homebrew game by a first time GM set in the Dragaeran Empire world of author Steven Brust's Vlad Taltos series, and we affectionately nicknamed the campaign "DragaeranBallZ.")
What some of them loved about the system and others complained about the system was the same, and I heard it time and again: "Every class feels the same, and they all feel like a wizard+[insert class]!"
In 4e perhaps the most dramatic change was that every class, including fighters, had increasing numbers of "powers" to choose from that were daily or otherwise limited abilities that could do things like "damage plus the enemy is pushed this many squares" or "damage plus you and someone change places" or "weapon damage plus [insert elemental type] damage" or even "deal damage to every enemy in a five foot radius and they are slowed" or perhaps something that didn't involve damage at all.
For me this aspect definitely had its charm in terms of versatility and classes like fighter getting in on a bit of "controller" role stuff like battlefield positioning, and more options often lead to more interesting tactics. I played a Warden and having powers helped relieve me of the unease of not having a long prepared spell list at my disposal. Also this guy I had NEVER seen play anything other than a mage or psion type spellcaster (and have not since then either) actually played a martial styled cleric for once. And he had a blast because he dealt excellent lightning themed damage and still had a diverse selection of spell-like "powers."
So I'm wondering, did others feel the classes were too similar? Or did they welcome the balance and/or variety powers provided? Or none of the above?