Posting this also here (also posted to video thread). I found this from reddit; if you are not sure how to play thief or want to get better, this person has some good thief guides: http://www.youtube.com/user/rainythief
He explains his builds, but more importantly he explains basic ideas how skills/traits work together. Builds will always change, but after understanding things you are ready to invent and improve your own build.
Here is dagger dagged backstab build explained. People seem to talk about this on every GW2 forum: