payment processing, reach, marketing, infrastructure, testing/approval (you may or may not like this, but users probably do like it), app delivery, etc.
The problem is that the value of these things goes way down the more crowded the app store gets. Seriously, what good is approval in light of the endless legions of crapware and clones? How useful is marketing when discoverability is almost nil?
And how about free apps? It seems to me that paid apps subsidize free ones but that's a whole other ball of wax.