[Oni]

Well a deface page is different for everyone.
Could be to say your security sucks, good thing I'm here before something really bad happened.
For others it can be like ha fuck you guys I'm deleting everything because you suck, kill yourselves.

To make one you have to decide what you want your purpose in it is, then design it like that.
Usually has a logo and a name. Most people use Javascript just to make it more interesting. Or it can just be a basic index.html

Check out some defaces and see what you do and don't like.
Here's AF's; http://www.anarchyforums.net/defaceindex.html

Ours desperately needs an update. Totally forgot about it.