Practically anything that is to do with creative Doctor Who. If you're posting a very long story, it's considered good form to split it up and number the bits. This will give your story a better chance of being read. This group is totally unmoderated. This means three things - you can quite happily post a story where various companions and Doctors do wierd and terrifying things with chocolate sauce to each other, and you can probably expect to get one or two rude responses.
Binaries are the main thing.
Other groups exist for that purpose. Someone comes up with the concept, say the Doctor must win a road-race, or defeat the aliens, or just figure out what the hell is going on. A crossover is a story which links characters from two different sources of fiction. It's usually courteous to list who the crossover is with to avoid confusion, although you may wish to leave it out if you want to keep it a surprise. A drabble is a story of exactly 100 words - no more, no less.