Karma could get out of hand, especially a thumb up / thumb down feature. It's vague, and a completely lazy form of player assessment.

What if a narrator thumbs down a player because he: doesn't like the player's style of writing / doesn't like how long their posts are / doesn't like how short their posts are? A thumbs down (or up) could mean anything, and tells future narrators absolutely nothing about the player's quality of writing.

The alternative?

Be an invested narrator. Search through your potential players' histories... and decide for yourself if you like them as a player. Vice-versa for players. Investigate your narrator's history, and make your own informed opinion.

Plus, I think a thumbs down option would just wind up encouraging unnecessary bullying - and deter creativity. It's why Facebook has stuck to the "Like"-only philosophy. You can't be abused by the simple click of a button.