Several reviews have mentioned that there is a lot of repetition in the game. Both in the fact that there aren't a whole lot of distinct enemy types, and that many of the quests take the same basic 'kill X of this kind of enemy' or 'collect X of this item' kind of structure. You're also running back and forth a lot between the same few locations early on. I haven't unlocked fast-travel yet but it will be a real boon.
It's a valid charge, truthfully, although whether it hurts the game is up to each individual. Very similar to Left 4 Dead, how one person might do each campaign once and then feel like they have exhausted what the game has to offer, and another person can play them over and over again because the variation in enemy placement, item drops, and co-op partners makes it a different experience each time. (I fall into the second camp personally.

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Other negative points I've seen in reviews are some graphics problems like frame rate drops and v-sync issues. In my opinion these are barely perceptible and not even worth mentioning. In my roughly 8-9 hours of playing so far, I caught one instance of v-sync trouble, and I honestly have not encountered a single frame rate dip in any circumstance. It's a really beautiful and fluid-looking game.
There are some major and minor bugs, from Achievements not popping in co-op games to some of the Siren's class skills being broken. These are real problems but I am assuming they will be patched quickly, since they have been acknowledged by Gearbox, and the fix for the matchmaking on the PS3 version came on like the second day.