OK, not really a one, but yeah, one. One unforgivable problem.
The meh: Not as witty as the first Portal, at least through the first half, but it gets better when GLaDOS, Wheatley and Cave Johnson really get rolling.
The graphics suck. Not the art, which is magnificent---the graphics. Blurry, generic textures everywhere you look, practically everything matte, half the time you can't tell what material they were going for, myopic distances. And I'm running a GTX 590 with an i5 2500, so I have the graphics options totally maxed, all knobs clockwise to the stops. The graphics are ported crap. The game should be called "Ported 2".
The bad: You buy the CD at a store, and when you go to install it, they force you to drink the Kool-Aid---you have to sign up with Valve's online store Steam, and their sales program tries to take over your computer. They even treat you to a pop-up when you quit out of the game. When you start Portal 2 from your desktop icon, Steam starts first. I will never buy another Valve title as long as I live, and I will badmouth them from beyond the grave if that turns out to be possible. That's why I gave it a score of one.