There are loads of opinions.
For me it can summarised in one statement - "They included Agro Management"
Anyone who plays MMOs will know what Agro, or Threat is. For the uninitiated/people with lives, it's a very simple (in principle) mechnanism in a computer MMO which controls which person a MOB (that's a critter/monster/bad guy) attacks. There's no intelligence behind it but the appearance is that there is.
Simply put - The MOB attacks the person it 'hates' the most.
Or to but it another way, it will attack whoever 'threatens' or 'aggrevates' it the most. So if you do alot of damage will attack you unless someone else does more for example. This is really only noticeable in group play of course. Computer RPGs will has classes specifically designed to create a lot of threat to ensure that the MOBs attack them; leaving the rest of the party free to do their thang. Agro managment is a defining part of MMO gameplay and understanding it forms the basis of every fight in the game.
In a pen and paper RPG (like D&D) the DM controls what or who the monsters attack. It's a defining principle of those games (and the opposite of MMOs) and a part of what makes them as special as they are. Like the fact you, as a player character, can try to whatever you want - it's up to the DM with the help of the rules that decide the result. That's why roleplaying is better than a board game in many respects; that sandbox approach, limited only your imagination. The bottom line is that you don't need to introduce agro management mechnics to the game, and you shouldn't. Real life isn't constrained by such basic things and never should a monster in a game. If a particular goblin really hates elves (because elves killed his family etc etc), then why should he be forced to attack some random Paladin when there's a horrible elf over there pewing pewing him?
But they did in 4E. Why? I don't know, but the fact that they did in the first place shows that the designers had lost touch with what makes a P'n'P RPG different from a Computer RPG/MMO. The mindset rippled down throughout the entire ruleset and is a perfect example of why 4E fails as a roleplaying game. The Agro mechanic was not absolute; they didn't screw it up that badly, but it was it was enough to make combat more forced, and less believable as a story telling element. That hurts a roleplaying game, and in 4E - killed it.
Note, I'm not saying it's not a good game. It IS a good game, but it's not a good roleplaying game, and that's why it failed.