I liked 3.0 and 3.5 when I first started playing them. There was a time when ignorance was bliss and everything looked new and shiny and awesome. After playing through a few games, we all learned how to customize, which spells were the win, which feats were the suck, and which PrCs really offered the bang for the buck.
Once several players learned how to really min-max and just how potent the Big 6 were, I started to not enjoy the game. As a DM, once the players knew how to "break" the math, I was forced to alter every creature and encounter. CR went out the window, and planning the games so that they were challenging without just being High-HP, High-BA, High-damage creatures became a beast.
And, with some players designing characters to be awesome, those players who were just creating a character for flavor and not worrying about power or balance, they feel very outclasses throughout the game.
I hope that the paragon paths and epic destinies cut down on the way players could use PrCs to break the game. I hope that part of the balance of 4e does not rely on players not realizing what is awesome and what sucks. I hope that the burst-type characters and swingy damage are minimized (I have had too many players have characters who, through design, can either dish out a ton of damage with proper preparation or stand around not able to do anything in combat - I cannot balance encounters based on whether the death star has a firing solution or not). And I hope that feats are more evenly balanced and useful so that there are no longer feats that are only taken as pre-req, which mainly meant that a later feat or PrC was punishing the player by making them expend a feat.