The modern one reflects some... changes - the more contemporary window style, and the number of minifigs wearing ties in the firehouse has increased. I expect it reflects an assumption of a higher tax base in legoville. In all seriousness, the custom piece count is on full display, where the 90's set is dominated by reusable generic pieces.

Those outward-sloping windows in the 90's set were the bomb though, useful in so many cool applications - (star)ship bridges, towers, and the like. :D