For me, a good exchange should have a simple yet nice website design, fast respond to customer support, friendly staff, secured wallet, and low trading/withdrawal fees, I'm a small-time trader so I want to save any profit I got from my investment.
Everything is here already and one thing that i want to add is that, they must be more transparent so the trading volume is real and not just a fake volume. Many big exchanges are being hacked so security must be there top priority since millions of money are on their exchanges. They don't need to become the best exchange, they just need to be consistent on delivering a good and quality system thru their exchange.