I posted this elsewhere but I suppose it belongs here as well. Mike Mearl's stealth answer from the Escapist Magazine interview from August 21, 2014.
Interviewer: "Do you have any other examples of what you think of as the DM’s power and responsibility?"
Mike Mealrs: "Our rules for stealth, which may sound like a funny example. But having worked on 3rd and 4th edition, creating a set of rules for hiding from other people and monsters that run without a DM, is crazy. You always end up with a situation where you’re standing right in front of the monster but he can’t see you, because there’s a loophole in the rules."
"So we just came out and said you know what, let the DM decide. We’re going to tell you the mechanic and just say, look DM, does it make sense that a player can hide in this situation? If so, let the player make the check. If not, don’t let him make the check. If maybe, then maybe advantage or disadvantage, that covers the middle ground."
"There is this funny thing that happens, and stealth is a good example of this. If you want to make a rule that is DM-proof, you end up with a rule that when humans try to read it, it just seems really weird. It’s like the old Carl Sagan quote from Cosmos, “If you want to create an apple pie, you must first create the universe.” I just want to make an apple pie, why are you describing how to make a black hole? Because this is way beyond what I need. So the rules just take on this tone where it doesn't seem like what’s actually happening at the table."
"Instead when you rely on the DM, it’s more the human element, and the rules just seem sensible. You can hide, when people can’t see you. Of course, if someone can see me, then how can I hide? It just seems like common sense. Where, when you’re like, use the grid, and here’s the different gradations of cover, it ends up introducing all this jargon. We can take the simplest concept, like trying to hide, and turn it into something which looks completely alien to someone just reading it."
