I look for all aspects of players. I use a set of house rules for my campaign and currently have five players in our group, but willing to make room for one more. Never know when you will lose a player to circumstances beyond control. Our group consists of a rules advocate, a combat strategist, a role-player, a newcomer to the game entirely, and a puzzler. A good mix in the group, challenging none the lease.
I have over 25 years experience with DMing and playing all editions of D&D except 4th edition, and have learned from many experiences from being a DM at game conventions, Gen Con, and numerous campaigns I have ran in the past. I keep up on things that can make a game fun and a great experience for the players.