Yeah, the all-seeing spellchecker is fantastic (control-click in a text entry pane, and select Spelling->Check Spelling as You Type).
To me, though, as a user in both "worlds" (work=PC because I must, home=Mac because I can), my favorite thing about the Macs is OS X, and my favorite thing about OS X is how it makes everything seem like it's all one happy family: yes, of course, I use many different apps and OS components, but they all seem more coordinated on OS X than under XP. In geek-speak, I do less context-switching in OS X, and I love that.
If only OS X would let you resize windows from more than the lower freaking right corner (a big plus in Windows' favor - and in nearly any other OS's favor), it'd be darn near perfect in my eyes.