Fot me the most important thing in exchanges is the reputation, I saw a lot of exchanges being hacked, so now I only stick with the high reputation exchanges like bittrex and binance, good customer support also an important point, low transaction fees and a lot of good coin to trade
Bittrex has included on the list of worst exchange in this year, Look at the daily trade volume of bittrex and how it goes even lower than before. So many people have problems with it and that make all of them are migrating to the binance and okex. Reputation is a good thing but the main thing is about how they can manage their customer to provide the best service.
Other things that attracts me to other exchanges include the withdrawal fees, I will make sure that they have a low withdrawal fees and also the security, I will make sure that they implemented enough security challenge and kucoin is one of such exchanges.