Speed, upfront underwriting, light stips, good communication, next day commissions and most importantly being honest about what files you'll be most competitive on with a broker upsell. I've encountered a lot of funders that say they have an "A paper" box that requires 600+ FICO score and 2 years TIB, but the rates and terms they present are B and C paper... If you're not going to be competitive in certain boxes let your ISO's know that. A great way to do this is to have an ISO package that shows the minimum requirements, buy rates and max terms for each of your boxes. This would save a lot of time on your end as well by not underwriting files that have no shot of closing, and keeping declines to a minimum.

From an ISO's perspective it looks like funders that don't do this are trying to backdoor you by having you send all of your files to them. Which reminds me, always give a thorough explanation why you passed on a file so your ISO feels comfortable sending you more files in the future without the fear of getting backdoored, which is everyone's biggest fear.