When there was only Universal Studios, I always found it a case of paying $35 just to go on the Back To The Future ride. I felt like majority of the park was all a facade. There wasn't the feel of realism in the place, like MGM. IOA is a great park with some pretty decent rides, especially Spiderman, but, for me the real difference is the staff. I find them really really unhelpful and quite often unpleasant. I think that this coupled with an apparent lack of real atmosphere (there is always an excited buzz in the Disney parks) mean that IOA really fall's short of any of the Disney parks. Sure I'll go to visit them, but there is really only once place to go , and we all know where that is!
I agree with Mr Lightbeer in that competition is good. You never know, if Universal had not sprouted up, would WDW have evolved so much and become what it is now.