I love Rammus to death, but I do feel like a few of his skills are lacking somewhat.
Powerball is staggeringly good--I daresay it's one of the best moves in the game period.
Ball Curl is exceedingly effective as defense, but again its reflection is unnoticable. If it calculated reflected damage BEFORE armor, it would actually hurt. In fact, it might actually hurt too much--but it could just reflect a lower percent. I just think it's entirely contradictory to have a character that you want to make as tough as possible, but doing so makes one of his skills explicitly worse.
Taunt is fantastic, when it works. If they can get it to reliably work, I'd be fine with it. The armor reduction is nice, too--helps it be a more team-aware skill.
Tremors, I think, ought to do a bit more damage to people, but leave it where it is on buildings--I think having a character skilled at taking down towers is very cool, but I'd like to see people react at all when I'm tremoring at them beyond level 7. I don't want it to be Crowstorm-level power, because Tremors doesn't have a windup, but just a little nudge would be perfect.
Also, his passive is one of the coolest and most awesome in the game, don't touch it--it's just too great, and a great way of encouraging tanking without requiring you to weaken yourself offensively.