Also make sure you fit within the pre-defined party setup that you're queuing up for. If your formed party is the entire group (4 members for most dungeons, 8 members for raids, etc), you can do whatever funky group setup you want. But if you want DF to fill in the rest of your group, you can't do strange stuff like queuing up with 2 tanks for a 4 man dungeon, stuff like that.