I already mentioned enrage; but there are a few more special melee abilities that monster can have.
Some monsters can flurry: that means that they can once in a while perform several more attacks on their target than they would normally. Flurry is a melee proc, so its rate depends - among other things - on the monster's dexterity, which means that debuffing dexterity makes the monster flurry less often.
Some monsters may either completely clear their aggro lists or just forget about one person from the top of their aggro list. This will make the monster attack someone else, perhaps a non-tank. There's nothing that can prevent this; the only way to deal with it is for the tanks to regain aggro as fast as possible. This monster ability is however rare, only some of the boss monsters have it, and it will be clearly mentioned before we engage.