I think there are a few things that are key* to successful adventure paths (APs):
1. Not too long. They are really mini-campaigns that most can see actually finishing in under a year play time (assuming playing at least once every 2 weeks), so they are not very daunting to both GMs and players.
2. Pre-made connections. While there are many modules that I love, and there have been some trilogies (U1-U3, Saltmarsh series is one of my favorite of all time, and all game systems), hacking connections between modules can be a pain. With these, it is already done.
3. Expanded world information. As mentioned before, each adventure path also opened up a section of the world with more detail, so while you were playing through the adventure you were also learning about the setting. Very cool that, I think.
4. Production quality. Hands down, Paizo has become one of the leaders (if not
the leader) in RPG production quality. Pretty maps and pictures (although I am not a fan of their elves - ears are too large/pointy for my tastes), full color, glossy format, even the smaller modules that end up being a mere 15 pages, all contribute to being some very pretty books. I had to force myself to end my AP & module subscription (it was dang near physically painful) when I started to save money to buy a house - even though I NEVER PLAYED/PLAY Pathfinder. I just loved them that much.
I think that Shadow World is prime for such things (adventure/settings combined), and I also believe that the first one should be in Quelbourne, Land of the Silver Mist: you have a very lightly explored frontier area where it is easy to say the PCs have just arrived at (for whatever reason/s) so they don't know this area and will be discovering it as they adventure, hostile races and cults, an ancient city being searched for, etc... In other words, several different peices/parts that can be melded together into an AP. (One of the only things I don't like is that both the Monks of Shaaljin and Rebels of Galtoth were human cults. I preferred to make them into the non-human races from HARP, the Kachenjunga and Wendigo (respectively). I just think that Kulthea should embrace the high-fantasy feel and not try to turn itself into a pseudo-middle-ages fantasy setting.)
*Other than just being attached to a very popular/successful game...