It ended up that customer service ended up being pushed as the differentiator (the interface I can get with Mint), but I've had great customer service experiences with both Ally and Charles Schwab. And I get all those previous features I wanted, plus things like ATM refunds. So Simple ended up being a feature-bare product in comparison.
I think their customer growth tailing off was a direct correlation to their feature launch velocity tailing off, and I attribute that to the legal stuff around banking being really hard, rather than any slacking on their part. Hopefully the purchase will allow them to leverage more existing agreements to get more features up and running.