Fiddly and vague, two things I don't like in rules. Perhaps the DMG will clarify at greater length. Fiddly, vague and long, three things I don't like in rules. It makes sense from a normal perspective, I'll give it that. Sneaking around is hard. Still, I appreciate the understanding that this is by and large at DM discretion. In the clash of battle, how easily an opponent might see you slip into the underbrush or or if they might even hear you over the sounds of shouting, screaming and swords clashing. How well one might smell you in the rain (a light mist can do a lot of obscure the senses beyond sight alone), are all up to DM direction and I appreciate that, though I feel like if you are going to make rules so intent vague as to basically not have them, well, I don't like that.
However, as is the case with most pre-4th styled rules systems, these things are often easier at the table than the rules make them out to be, and I suspect this will be the case with Stealth.
One thing it does call into question is facing. I haven't seen rules for facing but the rules for Stealth basically imply that facing exists. An opponent whom who are substantially behind obviously does not know you are there, or if they know, will have an increased difficulty in tracking you if you move and may be unaware of your specific location at any time. I would almost be tempted to argue that we would need to count 180-degree or greater turns as part of the distance you can cover during your move action.