I was pointed to this interesting article today:
http://cbs4.com/topstories/topstories_story_219064717.html
According to it, only 40% of the space shuttle launches are on time. Interestingly, the number one reason for delays are technical issues. They are to blame for about half of the launch delays. The weather, which I thought to be number one, is actually the second-most reason. About a third of all launches are delayed due to bad weather.