I really like GDAX and Coinbase due to FDIC and USD pairs. I don't like the limited amount of coins they trade. Have you heard any rumors about them expanding the coins they trade?
A while back Brian Armstrong was asking on Twitter for suggestions for new coins - don't know what came of it.
GDAX/Coinbase are very conservative, they don't like the idea of delisting coins, so will only list coins they think will be there permanently. Look at the struggle Litecoin went through to get listed!
I think the criteria for new coins is a) huge sustained volume, b) innovative developers c) real evidence of adoption
Not many coins fit that bill