Security, Reputation and Fees is what I went for.

Security is really no.1 although you can never really be in control of your funds on an exchange
if we can look after security of login ourselves it is best to have good security in place in case our hardware becomes stolen or compromised.

Reputation is important too, the last thing anyone wants is shady platforms and there is a lot of chat about various exchanges whether justified or not I'm not sure.

and fees of course, no-one wants to pay big fees, although we all know fees have to be paid to maintain a proper service for all to use in the future.