Talk to me about Keith Jarrett

I accidentally stumbled across Keith Jarrett’s Koeln Concert at a dinner party the other day – lovely, great, fascinating music. I hadn’t heard about Jarrett before, more’s the pity, and now I’d like to know more. I’m looking for people to tell me what else to listen to now – where should I start? What should I be looking for? What’s not so great?

Enlighten me, please.