Not sure if "Off-topic" is the best place for this.... maybe "Trading Discussion."
Anyway, first and foremost, a real-time, non-lagging trade engine. Non-buggy platform is a must. Easy-to-use interface. Trading options -- stop orders, please. Regular auditing. Good cold storage protocols. Fast fiat withdrawals. More, but not off the top of my head.
Thanks for the input. Although these features are more like a general guideline for any exchange to follow. Maybe some more obscure things?