The hacking in Deus Ex Human Revolution.
Now don't get me wrong, its actually quite fun, and they managed to make it not wear you down like Bioshock's hacking system. But. This sucked a bit of the fun out of other parts in the game because hacking was everything in that game. You hack doors, safes, computers, alarms, security panels, etc., and it means that without hacking, you only get to see about half of what the game has to offer, which is a damn shame. I really wish they had at least kept lock-picking in from the original. It could have been a simple system, and it could have at least made some other options viable.