Success or money.
I used to think money but I had a discussion with a self made millionaire who still does it and I kinda changed my mind to it being "success" but saying that I wouldn't do it if the money wasn't there....
This is the geezer... very interesting he is...
http://www.occamgreen.com/documents/...s_Orlovsky.htm
eg when u get a new site concept is it the thought of making a totally new concept or is it the thought of making the $$