A brontosaurus is thin at one end, thick in the middle, and thin at the other end.
Well, that's some of the story, but like most generalisations, it's a generalisation.
Other things that contribute to the oldskool feel:
Art (that changed, and it's continually underrated in terms of it's importance to the "vibe" of the game and the ideas it generates)
Monster selection (that changed...there's a lot of "flavour" staples missing from the core tome...e.g. where the heck is the leprechaun? Count the number of Dungeon magazine adventures that revolve around these little guys, then reconsider their usefulness beyond what CR they represent)
House rules (people seem to do it a lot less, gone are the 3-ring binders)
Settings (they changed...even the ones that are still about are perhaps approached in a different, non-pioneering way - e.g. Greyhawk may still be getting dungeons, but some of them are puportedly made by people who have no interest in designing a "tower of orcs")
Stats (NPCs used to take seconds to create, now you need a computer program to generate half a page of stuff...yes, you can fudge it, but that speaks of a design error which relates back to your original point - character design was made highly customisable for the benefit of PCs, not NPCs, which is why 4E could perhaps benefit from an abbreviated character creation chapter in the DMG)
Fashion (what's "in" now, and what's considered passe is different...dungeons have to "make sense" rather than "be fun", for instance...who would be caught dead doing a chess puzzle room like in Ghost Tower of Inverness or a puzzle dungeon like White Plume Mountain? Bedrooms, guard posts and empty rooms have replaced Fighting Fantasy-style weirdness...too many DMs & designers care what the kobolds eat and where the orcs go to the toilet)
That's probably just scratched the surface.