Bosses with predictable patterns.  Not because they are easy, but because they are BORING.  You just sit there, going through the motions, avoiding the easy-to-avoid attacks and waiting for the scheduled opening to dish out your attack.  Rinse and repeat.  The only way you take damage is by simply not caring after a certain point and taking shots because you know you're going to win this one.  So why put on a significant effort? 
 
Take the opening boss battle in Darksiders.  Wait for the boss to slam the ground, avoid the damage splash (how big a cliche is this one right here.  He'll do this twice, then reach into his lava pit and toss some cars at you.  Grab one, toss it at him (thankfully the game has lock-on otherwise there would be a challenge) and repeat the cycle one more time.  Hit him with two cars in a row, he'll fall to the ground leaving him open to sword strikes.  Then he'll get mad, blow some air (which again you have to avoid) and the entire process just repeats itself. 
 
Please, developers, stop doing this.  I know it's easy because it's just a matter of AI programming but my God, is it a drag.