Nothing, I don’t even consider them. Some are pretty good, but at the end of the day, there are only a few good brokers out there that you should stick with.
Some brokers make being an IB really difficult for us. Some of them will give you a form that will take me a week to fill just to get a referral link to refer brokers. What do they take us for?
Regulatory body / nationality is the most important feature for me. Since I live in Australia, an Aust broker would be essential. ECN would come a very close second. Once these two conditions are satisfied, I then generally look for the broker with cheaper commission / tighter spread.