A good exchange for me consists of the below qualities:

-User friendly interface
-Bugs fixed and websites/apps being updated for improvement
-Minimal downtime - Very important if you are an active trader and there is a sudden surge of trades coming
-Responding Support (Chat must also be available and could address your concerns as we are talking about money here)
-Technical graphs should be able to be modified in order to plot your own strategies and apply different studies/parameters
-Have minimal charges (of course who does want to have high charges especially if you trade from time to time)
-Good servers that would be stable as it is very frustrating to not be able to trade real time (I experienced on missing to sell at a high price for a good profit because the server can not handle the surge of trades happening)
-High security verifications