Single stock trading sold as an investment product to the low-end of the retail market (i.e. people like me who's limited investment dollars should be dumped into passive, diversified funds.)
If all RobinHood users followed sound "buy diversified ETF and hold" investing advice, my understanding is that they'd go bankrupt.
If I had browser data from everyone on RobinHood, and was able to track which stocks they were looking at, I might be able to make money front-running.
If Robinhood is incentivizing or encouraging frequent trading by users, that's not really good in my opinion. It doesn't make them worse than other brokerages, but it doesn't make them better either. And with a name like Robinhood, they are positioning themselves as white knights.