Personnally it's all the small feature that make a Mac so wounderfull. Exposer it's probably my favorite. Dashboard is usefull, spring loaded folder, spolight, iLife make it easy to small or personnal project. One thing I really like is the Unix subsystem, you can run apache, php, mysql, webdave, ftpd, SVN server in no time. You don't like one shortcut in a application, change it in system preference, you don't like the position of a button on a cocoa appz, open it in Interface builder and change it. Want to do an automatic task or a repetitive one: automator, applescript, cron job, shell script are all there for that. Having many machines want to sync them, rsync is avaible. Booting from an external drive.
For a programming point of view, OS X rock, especially that the new machine can boot Windows, you can install KDE desktop directly into OSX to switch from Aqua to KDE.
The drawback now: expensive to updgrade machine, expension limited except on MacPro, some printer indeed are a mess to use (check your model before buying one) but HP Epson and the like shouldn't pose problem. Game are defenitly the major drawback if you're a gamer, but the dual boot save the day here. Few appz is missing: Autocad, Catia, 3DStudioMax... there's equivalent but not the real appz.
I strongly sugest you a good launcher to replace the dock launching (Quicksilver, Dragthing, drop drawer,...).