I agree with klahsipper; something flexible, and something driven by active people, who write up plot memos and keep constant updates with everyone on what's going on.
My biggest need to see in a TP? Weyr-wide involvement. The best TP's have always been the ones that actually almost require that everyone participate. If there's a trade-development between Ista and Fort, it's not going to effect Joe Schmoe kitchen worker. TPs can't just involve the higher-ranking members of the Weyr, or even JUST the lower-ranking. Something that effects everyone has a higher probability of working as a great TP.
Good TPs should either center around drama or comedy. Politics are, honestly, boring. Mystery? Even more boring. You ran out of supplies? Eh. Things keep disappearing around the Weyr? Probably not going to it it off. While I would LOVE to see relation problems between Weyrs, Holds, and Crafts, it's not something that should be a short-term fun RP. A deadly disease, wiping out everyone's family? Points for drama. A 'game month' where half of the Weyr is pitted against the other half? Comedy points. It has to be something that people go "OOOO! That sounds like FUN, and will help me build my character!"
If it doesn't effect their character enough to cause an impact, it won't really fly.
Gathers? Fun in principle, fun to read in books... Not fun to go to and play at. Sorry to everyone that likes them, but I admit to not attending them any more because they're all the same. Look at booths, chat with random folks, dance on the dance floor, watch a show at the end. It'd be much more fun if there was haggling with shop-keepers, a little devil pickpocketing everyone in the fair, a drunk ripping down tents as he rambles about, a knife-fight breaking out... Maybe I'm just not easily entertained, haha. ^^