Unsurprising, neither Microsoft nor Apple write good software for each other's platform. iTunes and Safari for Windows are both pigs. Office for Mac runs slowly, but is still a better office suite than anything else, which is pretty sad, but I didn't buy the mac for word processing anyway.
Well, the UI for one thing. You have two choices: blue or gray. Which doesn't really bother me, I'm not a UI tweaker, but if you're a linux guy, you'd be used to having something like Compiz. But that's just my answer to that question. Perhaps BetaWar has more specific complaints.