Security. What security procedures do they require, KYC, additional verifications, 2fa, additional security and login system, how do they handle potential or real hack attack, how accurate and forward they are in cutting losses and getting back on track after a hack attack. Also, liquid and customer support available. All the things I've mentioned are present in Bitstamp and that is where I perform most of my trades. As a backup, I am using fox change which requires no additional verification and is quite handy if you urgently need to change your altcoins. This is an easy to use platform which offers instant change between altoins.