The linear nature of APs gives a feeling of sameness to the books. The veneer certainly changes ... gothic horror, jungle/pulp adventure, Asian folklore. And Paizo does do a very good job of grounding these APs in specific cultural atmospheres that wonderfully evoke their inspirations. For example, I'm a big Halloween geek, and I just went nuts over Carrion Crown.
However, it feels like every single map has a BBG at the end that has to be defeated in order to progress to the next step in the story. With 6 volumes in an AP, it seems to me like we could be getting a lot more variety in structure.
Interesting OP. I started a rather similar discussion on these boards more than two years ago. Certainly interesting to compare the two discussions, now that Kingmaker has come to pass. You will note that even there Paizo couldn't resist the BBEG ending, even if the structure leading up to that was a refreshing departure in many ways.
I think Paizo has since experimented a little with the Adventure Paths veering occasionally into sandbox style (as witness the opening of Serpent Skull), and certainly Carrion Crown is an interesting experiment in its own right to adjust the overarching structure towards making it a bit more episodic like Rise of the Runelords was. So the experiments continue, and it's a very interesting ride.
On the whole, though, my impression is that Paizo is at the same time trying to play it safe and not alienate too much or too many of its customers who have come to appreciate adventure paths for what they are. It's after all a subscription model, and so you want customers to have some reliability going in as regards their product expectations. I say that because every time Paizo tries to deviate from the main model ever so slightly, we get threads saying how people didn't really like that.
In that light, looking at a posting by James Jacobs in this discussion upthread, it seems to me that Paizo has hit on a rather good solution to pacify some of those wishing for more structural deviation in the adventure paths - introduce a different product line to cater to such wishes, such as the Campaign Setting line.
All said, I'm curious to see how this works out. So here's to another interesting thread in 2 years time!