I was away from crypto for years and now I'm trying to get back into the game. I was really STUNNED that there is no exchange, even after many years since bitcoin started, that we can just deposit USD and start exchanging altcoins. I hope I'm wrong because this would be the single biggest hurdle for altcoins to be widely adopted by general public. There are just too many hurdles to jump over just to START trading.
The best exchanger for me is poloniex, i had try with other ones, but i always end back in polo. And i like it for the next reasons.
Security: User should have access to his mail for any transaction, they give the option for 2fa, and it have not a bad feedback about hacking history.
Time: They don't hold users coins, when you make a withdraw they process it fast.
Limits: Without a verify account, users can withdraw $2k/day, that isn't bad at all.
I like the exchangers who let you feel you are holding the money. sites like bittrex can be a bad experience.
By the way, if shapeshift jut had a nice fees and not a nuts one, would be the number one for me.