No need to look further, we have big exchanges with high liquidity so no need to look for more. I personally use binance, bittrex and hitbtc.
I list some though these are common things that we should look up to.
Privacy
Security
Liquidity
24/7 support
Instant deposit and withdrawal
To this list I would just add low taxes for exchanging, deposited and withdrawing. That's also important term for every user. Honestly, I check the price for taxes at first, before to register on an exchanger. Luckily this info is available on almost every exchanger without registration.