...Like, say, the scorn of your friends.
In the spirit of Twitter, I'll make this brief. If you're running a Twitter contest or promotion, then please - I beg you - have entrants do something more useful, more conversational, more interesting than just retweeting a link to your latest sale item or a message about how fabulous you are.
I thank you. Your participants' followers thank you. And at the end of the day, your brand reputation will thank you.