i think one of the major reasons the mac is better for graphics is also one of the most shallow - because graphic designers use macs. there is the pool of compatibility there, with software, with print houses etc.
i have to say i converted to mac purely because of the argument: graphics is better on the macintosh. to be honest, it isn't, not really. photoshop is the same photoshop on windows etc, all the software is the same, but the fact that the mac is a better platform completely, not just from a graphics POV, makes it better for graphics.
as mentioned before, expose is fantastic for graphics. the third option, "show all windows in current application" is a godsend.
example:
you are working on an image. you save it but carry on working, placing an idea into it you'd just had, and the work starts to take on a different appearance. open the save before you started developing this idea, and hit F10. both images are then placed, perfectly scaled and the same size, next to each other. now you can compare, and decide which way to go. it's not even like it's an intensive feature, i have it set to the bottom left hotcorner, so i just throw my mouse to the bottom left and there's all my photoshop windows (i've had 20+ images open at once before).
there could be 10 more in illustrator, 10 safari webpages open, iTunes, iPhoto, iCal, numerous Finder windows, 10 messenger conversations, and the system is still stable.
you keep all your camera images in iphoto. you can drag these images onto the desktop, copying it, then drag it from the desktop onto the photoshop icon, opening it. you edit it, save it then drag into your director project. along with music you just dragged from iTunes. you save it as a Flash file and drop it onto safari to preview it. you can drag from one app to the next using expose, show all windows (gets fun with the amount of windows open mentioned before (50 windows all open and tiled and scaled properly....) )
you get the feeling you are using a powerful tool, unlimited but to your action. with windows i always felt i was struggling against the computer, not working with it.