Graphic design: not many. Just little things, like the great desktop zoom, the nifty screenshot shortcuts, simple access to special characters. Nothing that Windows couldn't do without some modifications, just maybe not as nicely. Also, I spend most of my time in OS X anyway, and I don't want to boot into Windows just for quickly creating or editing a file in Illustrator or Photoshop.
Browsing the web: font rendering. I don't like Windows' aggressive hinting and the one dimensional anti-aliasing. Some non-standard fonts I find not only ugly, but unreadable on Windows.
Webdev: Unix underpinnings, Rails. Text rendering, again. And I feel like I'm a lot faster at switching apps and searching for stuff on OS X. But that's probably just habits I've built over time.
Overall, OS X gets in my way the least. That's why I would choose it if I had to choose just one OS. But I don't, so …
Browsing the web: font rendering. I don't like Windows' aggressive hinting and the one dimensional anti-aliasing. Some non-standard fonts I find not only ugly, but unreadable on Windows.
Webdev: Unix underpinnings, Rails. Text rendering, again. And I feel like I'm a lot faster at switching apps and searching for stuff on OS X. But that's probably just habits I've built over time.
Overall, OS X gets in my way the least. That's why I would choose it if I had to choose just one OS. But I don't, so …