Now I know this would be hard to implement, but I think it could look really cool; you could spot grizzled old veterans easily by the dents and scratches in their armour. And n00bs would stick out like a sore thumb, even in their ebayed FoW, due to being all shiny and new.
The same idea could be applied to weapons and shields too.
Would be very hard to implement on the other classes, but the GW artists are so talented, I'm sure that they could come up with something

Of course, some people would want their armour to stay eternally shiny, so make it a tickbox option like the titles, so it can be turned on and off.