Dungeon Siege. I have played Diablo II over, and over, and over again, and yet I find DS boring because it's an incredibly tedious click-fest.
What makes DS boring and Diablo addictive I don't know, but it probably has something to do with the fact that DS frequently forces your characters to stand in a circle and then get raped from all sides by way of cutscenes where control of 'your' party is taken out of your hands (*edit* I'm not talking about a circle formation, by the way, I'm talking about a circle. The wizards don't stand in the middle of the warriors, they just stand in a line, get engaged in melee combat and get promptly buggered.)
Would Total War games have been better if at the start of every battle you got to watch your army march to the bottom of the lowest hill for no reason? Would beat-em-ups be more fun if your character started out by turning their back, grabbing their ankles and pulling their trousers down? No. So who the hell thought this sh*t would be a good idea in an RPG?
I've no huge objection to monsters spawning from all sides. But no amount of suspension of disbelief is going to make me believe that battle-hardened warriors will, upon seeing a creepy-looking necromancer in the middle of a stone circle that screams "this will become a closed arena and remain so until you kill the guy in the cape", break formation and wander randomly into the middle of said trap.
Honourable mention goes to Deus Ex 2. Deus Ex showed that games could be as thoughtful as a good film, and even some books. Deus Ex 2 showed that the moment that happened and such a game sold bucketloads, someone would come along and make a sequel devoid of absolutely everything that made the original great.
This is possibly a bit unfair as I never played it for very long. Albeit that's because both times I persevered, the game crashed and corrupted my save game.