There are two main reasons I didn't like Ocean's Twelve. First of all, it's entirely pointless. It has no heart, drifting instead between several plot lines that do tie together, but not in anything you'd call a climax. There is a "big" heist they have to execute, but is far from central to the film. Most sequels at least try to be bigger and better than their originals; this one lacks even that ambition.
Secondly, the use of cinematic shorthand to tell the story really bugged me. There are lots of knowing glances, and histories alluded to with a nod and a wink. It lends the illusion of depth to the characters, but when the credits roll, do you actually know anything about them? No. This shorthand extends to many key sequences in the film itself, which are not shown as part of the action, but instead revealed after the fact in flashbacks or conversations.
As an ensemble star vehicle, it is appropriately self-indulgent and deliberately harmless to each participant's position on the Hollywood status ladder. But as a heist or a con movie, or even as a crime comedy caper, it is utterly flaccid.