I honestly haven't done a ton of studying of late and things change quite frequently given how early the industry is and how often the laws have changed. This page has some good information to get you started:
http://www.therealestatecrowdfundingreview.com/how-we-created-the-site-rankingsCrowdStreet is run by brokers, which I dislike. They seem to know what they're doing, but make sure you read through their documents.
Equity Multiple also seems like they know what they're doing, but the same caution applies. We ran one of their deals through our underwriting about 6 months ago and it failed to meet our expectations for a number of reasons.
If you start with Ian's top 20 or so sites here are things I'd look for:
1. Co-investment by the platform; preferably with their OWN money and not money from investors into their operating entity
2. A decent business plan for their platform where they can demonstrate how their operations cover expenses in the near-term
3. People from the sponsor industry that actually know how to construct deals and/or underwrite sponsors
4. A business model that will attract high quality sponsors and eliminate middlemen if this is desirable. This is almost impossible with a marketplace-only business model
So basically:
-Skin in the game
-A viable business model
-Experience as an operator or in scrutinizing operators
-Some features that attract good sponsors