For me there are two things that are most important. Primarily it's the fee structure of the exchange, for traders fees will eat away a large amount of your profits. Secondly it would be the UI, many hours are spent using an exchange and it being clean and efficient to use is hugely important.
Yea Fees and UI it's also something to consider. Day traders could be probably the ones able to say if the eyes hurt ..
Well you have to realise that at the end of the day traders are looking to profit. If the exchange is laggy or buggy for example that's costing them money. While dealing with issues moving funds or such also is costing them money it's a much easier thing to avoid.
Side note, anyone staring at a trade screen or any other screen all day, get yourself something like Flux so you can dim the blue light.