Definitely the brand. Not necessarily rarity as Gibson do pretty large production runs. Better wood, better pickups, (usually but not always with Gibson - I have seen a couple of gibsons that were absolute hounds) better build quality and finishing.
So the materials used by Gibson cost more. The finishing should take a bit longer. And you definitely pay a mark-up for the name on the headstock.
Maybe an even more accurate contrast is between a Gibson standard and a Gibson custom shop. Say the custom shop is about twice as much again (they usually retail at £4000 - £5000 depending on spec). Materials are usually pretty similar, although some of the woods used are absolutely top end and cost a mint. The difference is the guitar is hand-made by the absolute best luthiers. Sounds a bit more similar to the craft beer movement.
For the record, I would sell a kidney for a custom shop Gibson!