The ability to display anti-social tendencies by virtually killing people.
JK. I'd never touch any online social networking with a ten foot pole after deleting my FB/Twitter accounts 2 years ago. It's just not for me. Last.fm was good for a while (2006-2007) before it became a haven for snobs to display their snobbery and lambast anybody who listened to something they deemed inferior.
What would 'dislike' button really give you? It will be another retarded feature anyway. Give us better spam control, custom filtration of posts and news updates and organised data management.