A good exchange in my opinion should have the following features if not more, what the above people suggested:
1. A responsive support system
2. Quick notification system for downtimes, new coins/markets listing, new features.
3. A UI that is easy on both PC and Smartphones. Having an App is secondary for me, though others may want it.
4. Coupons, referral system and incentives to keep the traders happy from time to time especially the loyal traders and maybe the ones from bitcointalk.
![Grin](https://bitcointalk.org/Smileys/default/grin.gif)
5. A liquid market would be great but it takes time to get one.
6. KYC will probably come in once at a time and because some criminals attempt to launder money from tainted sources and so on. So that will happen but I hope you can keep it aside for legitimate users.
Some users have suggested leverage, but I feel it is being way overdone by bigger exchanges and for a new exchange like yours which is being developed, this should be a late goal once the exchange itself is functional.