For some unknown reason it does that. It tends to happen only after playing custom games, which may mean it is a Warcraft memory management issue (not your fault). Most of the time I have had it quit by itself after 10-15 minutes, however this is not very practical - the time may also vary based on the map played or the amount of maps played, I don't know.
As there is no fix (yet), all you can do it press Command-M to swap to window mode before exiting Warcraft. From here you can right-click it in the dock and Force Quit it.
You can also press Command-Option-Escape to Force Quit it when it has frozen in fullscreen (I prefer this way).
Use either of these two methods - don't force restart your Mac. Doing that is bad for OS X and any other open programs. Only force restart your Mac if OS X has become unresponsive. As yet I haven't seen mine crash (purchased Feb 08) and Command-Option-Escape is very useful for any programs that act up (it's the Windows equivalent of Ctrl-Alt-Delete).