The brief answer is: brutal overacting. I just caught a bit of Austin Powers while eating with my son the other day, and it struck me once again: watching anything with Mike Meyers onscreen -- he wasn't bad in Shrek is, for me, like having someone tell me jokes for a couple of hours while constantly shouting at me, "That's funny!!! Did you hear what I just said? It was really funny!!!"
I was thinking of Johnny English while watching Austin Powers, and thinking that when your acting is much less subtle than Rowan Atkinson's is, you are really, really overacting.