There was an interview with Hideo Kojima in the Australian PS2 magazine a couple of months ago... he revealed a few major differences which'll set MGS4 apart from the other titles. In the earlier games, there's usually been a clear-cut A versus B fight going on. Naked Snake versus Volgin and the Cobras, Solid Snake versus FoxHound, Raiden versus Dead Cell. Sure there are the background conspiracies and the pervasive influence of the Philosophers/Patriots, but the actual fight has been pretty clear-cut... however, according to Kojima, there's be up to five different competing parties involved in MGS4 - so potentially, an A versus B versus C versus D versus E. It'll be interesting to find out who all these factions are... and whether all the 'heroes' from the previous games are on the same side.

I hope Snake winds up keeping the beard he's got in the MGS4 concept art. The moustache just isn't as cool, somehow. Not that I'm biased toward beards or anything

Emmerich doesn't look much older; the presence of Raiden's child is also evidence that the game takes place less than a couple of years after MGS2. That'd place it in... 2010? That'd make Snake 38. Being cloned from a man in his 40s, it's understandable that he'd look as old as he does. Even in MGS1, it was noted that he "hasn't aged well".