Wow, so that's effectively opening up new easy to get credit vehicles for unsophisticated investors. How could that go wrong?
Even if you are a professional trader you'll take a bath on margin over the long run. They are incentivizing what is effectively borrowing for gambling and that is their long term revenue strategy? Might be a great money maker but it certainly is playing with some really big barrels of fire.
"Three things ruin people: drugs, liquor, and leverage - Charlie Munger"
edit: The point here is not that Margin is a new service, yes every brokerage has it, it's that it is irresponsible to use Margin if you are not a sophisticated investor. Its even more irresponsible to push leverage onto unsophisticated investors.
Not everyone is going to understand the risks of leverage and if Robinhood makes it as easy as candy crush to trade on leverage, enough gamified users are "playing" stock market with real money and a margin call happens during a huge downturn, it will be exceptionally nasty.