I know what you are talking about. I used to have a 38mm SS S0 and have now upgrade to the 42mm SS S2, and I notice the screen looks a little different when viewed under bright light - the S2's seems to show this wavy pattern on the edges and it looks like the screen has "sunk in" a little bit in the middle compared to the edge. Of course when you view the watch screen at its profile from the side, this is not true, in other words, it is more like an optical effect/illusion.
My guess to this difference is, the S2 has better waterproofing and Apple has changed the way the screen is attached to the body so the reflections from under the screen edge is making this optical illusion.
Otherwise I don't see any problems with this other than it looks different from the S0 - if you never owned the S0 before I doubt anyone would care.