Smart Stay is a feature which uses the front facing camera to check if the users eyes are looking at the display and won't time out the display when the timeout threshold is reached.
I'm not trying to say here that "omg iOS is better", I'm genuinely curious as to when this feature actually makes a difference. Seems to me all it's going to do is suck up power running a camera. And the idea of having the front-facing camera on at all times is also a bit sketchy. It'd be like having my computer's webcam on at all times.
its one of those features you don't advertise though. I mean I wouldn't buy a device just for this but small things like this make people miss them when using another phone / platform. I guess its not the big stuff that build the brand loyalty but these subtle details.