I think the most important think to look for exchange is security level to avoid issue of hack , and second one is high volume to get high liquidity. Dont use exchange that has issue with withdrawal and has been hack.
This can be a good point of view before you trade on any exchange, because not all cryptoexchange is good to use for. Top exchanges can be a good place for newbies since its already proven by investors, but if you want to look for more then that is the time for you to make more effort. You must first look for the system of the trading exchange and if they are operating legally, and next read some reviews about that exchange because mostly all the disappointed user will always have their thoughts out in many social sites.
You have a point there, you can identify legit and good services exchanges if you keep research in here or you can visit at this thread,
Exchanges. There's a ton of complaining people there regarding an exchanges site problems or different issues that they encounter on exchange services that you can possible you can avoid in earlier.
Good exchange sites are: Having good customer support (42/7), high volume for possible high profit earn and the last is fewer fees if possible.