According to the TF2 Blog, game servers will now be scored so that players can easily see which are "good" servers and which are considered "bad" servers by the new scoring system. Below is the formula used to determine the score of a TF2 server:
This means servers that have lots of players coming and going quickly will have a bad score and servers that have players joining and staying on the server for a while will have a good score. I think it's a decent way to score a server and a way that can prevent "cheating" of the system since the time spent on a server is avaliable to TF2 by client, not the server. However - cheaters always find a way.
Head over to the TF2 Blog and you can find out more about the system.