I trade only on a few exchanges and my primary thing which I look for is security how secure is the exchange. Secondly, I look for liquidity how much daily volume is that exchange having. Thirdly I look for how frequently they list new potential coins to their exchange.
Each person has his own way of trading, I am not the kind of person that is investing in new coins that no one has ever heard of, so if an exchange is secure and has enough volume most of the time that is enough for me to consider that exchange as a platform that I will use because my trades are restricted only to the most valuable coins that we have in the market.