I help maintain about 50 Macs that get used and abused daily , and we have rarely had any system crash. Note the use of "rarely." I have seen it happen, but it is such a rarity that it's hardly worth mentioning. We get far more strange and quirky problems than crashes. As others have stated, apps do crash far more frequently (especially if they're made by Microsoft).
This is a hilarious video I saw on Shoutwire. Check it out. http://video.google.com/videoplay?docid=-6553260189868317794