A Note: I don't know anything about Golarion, the adventure paths, or all the third party stuff. I make this post strictly from the point of view of the Pathfinder "3.75" ruleset and the main books associated with the rules.
Audience participation. This is one of the main reasons I really like Pathfinder and Paizo. Instead of dropping a one hundred and sixty page splatbook on us every month, Paizo involves its customers in the process. Advanced Player's Guide, Ultimate Magic, and Ultimate Combat were each cut up into bits and pieces, then released for public consumption as free pdfs. People get the pdfs far in advance of the book's release date, play around with them, post on forums, and your comments and critiques get involved in the next round of playtesting, and all the comments and critique of that round get involved in the final product. We the player get to help the company make their product, we're given a voice to complain with or critique with that the company listens to. It is grand. A cynical person might say that Paizo is getting free playtesting and errata work out of this, but fuck, I get free pdfs prior to a book being released, and if I want to, I can have an impact on a book. I think it is a fair trade off.
Pdf sales. I love this. I'm not going to lie and say I've never pirated a pdf. I love to own books legally, I try to buy as many as I can afford and find, as long as they are of decent quality. Pathfinder actually sells quite nice pdfs of their products, usually around the ten dollar mark. The pdfs are high quality, some with hyperlinks to other relevant sections of the books, and also always with nice bookmarks. I love physical books, but I don't need physical monster manuals, and a pdf option is a cheap way to preview a book you're curious about that you're not sure you really need. The pdf sales also show that Paizo is intelligent and understands that this is the digital age. Some people do all their gaming over the internet or on their iDevice or phone or whatever, and sometimes it is more practical for people to have a ten dollar pdf than a forty dollar two hundred and fifty page hardcover book. As I said in a previous post, I used a pdf reader and dice roller on my phone to build a scenario for Labyrinth Lord. Pdfs are handy, and I think most gamers are willing to buy, rather than pirate, if the quality is there.
Price. To talk about pricing, I have to rag on Wizards/3.5 a bit, sorry. Wizards is the king of the one hundred and sixty page splatbook for twenty-nine ninety-five retail price, page one hundred and sixty is always an ad. They have said before that they offer a reduced price on bigger core books (PHB, DMG, Monster Manual), so they can charge more on slimmer additional content books like Complete Arcane or Psionic Power or whatever. The price per page of those splatbooks comes out to nineteen cents per page. The price per page on the PHB and DMG is ten cents a page for six hundred and twenty combined pages. The Pathfinder Advanced Player's Guide and Gamemastery Guide are both over three hundred pages and they both retail for thirty-nine ninety-nine, ends up being about twelve cents a page for both of them. Ultimate Magic, which is close in tenor to a splatbook, is over two-hundred and fifty pages and weighs in at about sixteen cents per page, retailing at thirty-nine ninety-nine. The Core Pathfinder book is sort of a PHB and DMG combo of over five-hundred and seventy pages and is about nine cents per page, retailing at forty-nine ninety-nine. Granted, the last page of each of those Pathfinder is an add, just like Wizards' books. I guess what I am saying is that all of Pathfinder's core books are more expensive per book, but cheaper per page, so there is more content overall. Maybe. I dunno, the one hundred and sixty page splatbook page count limit infuriated me. I have never been displeased by buying a Pathfinder book, so I think that if the quality is there, I am ok paying forty bucks, retail.
Options, like 3.5 Edition, there are a lot of feats and classes and such and they give you a lot of variety. I'm not going to go on at length about it, I already have in 3.5's thing. One thing I do like is Archetypes. Instead of making hundreds of classes and prestige classes, Pathfinder has things like Archetypes, options you can choose that replaces a class' feature with something else that changes the way it plays. An Alchemist can exchange his Mutagen ability for an alternate Mutagen that improves his mental stats instead of physical. An Archer Fighter can replace his armor abilities to gain the ability to make trick shots with a bow. All of the changes are small, but they are effective at altering the class in a fundamental way to fit your goals for playstyle.
Class variety. Even without archetypes, the classes have a superior variety over previous iterations. All Barbarians will rage, but they now have variations on the effects of their rage. Some become terrifying, others develop an utterly impenetrable mind, others gain the ability to hurl huge rocks or cause their mount to rage alongside them. Sorcerers have various bloodlines that grant them differing bonus spells and feats and strange abilities.
Power creep. Power creep is a real threat in these games, latter 3.5 splatbooks are much more potent than the core book stuff. Pathfinder "solved" this by just amping up the abilities and powers of the base classes and races. This is an admittedly crude solution, but it shows that they recognize that power creep exists and are at least attempting to rectify it/head it off.
I really like Pathfinder's skill system, it is mechanically no different than 3.5's, but it borrows some elements of 4th Edition. Like 4th Edition, some skills are folded into others, Listen and Spot are now Perception and Move Silently and Hide are now Stealth, and so on. In addition, cross class skills are not a thing in this game. You have class skills, and if you put a skill point in one them, you gain a plus three bonus to skill checks with that skill. Buying ranks in any skill only costs one skill point, regardless of whether or not it is a class skill, class skills just get that nice plus three bonus. It is a little simpler to manage, you don't have to mess around with half ranks and that sort of thing, plus the overall list is slimmer, which makes picking and choosing a lot easier.
I guess everything I said about 3,5 Edition applies here as well, the games are very similar. The primary differences are Paizo's style versus Wizards of the Coast's style and a few slight rules clarifications, alterations, and so on.