You will notice a green bar underneath your profile picture.
This is as a result of Dave turning on the reputation system. You can add to someone's reputation by clicking the reputation button (looks like weighing scales)underneath that bar in the specific post for which you want to add to someone's reputation.

This partly overlaps with the thank you system. I am not 100% sure where you'd use the reputation button instead of the thanks you button.
Possibly on posts where someone shows off his skills or knowledge.

What do you guys think of the rep system, and how it compares to the thank you system?