Do you work for Paizo or something?
No, I don't work for Paizo; just a fan. For my day job, I'm a librarian in North Dakota.
They took the 3.5 rules and reprinted and sold them with very minor adjustments.
The changes to the rule set are rather more extensive than you suggest, though I'll grant that it's hard to see the overall effect because many of the changes are quite minor individually. Taken as a whole, though, the system is a lot smoother.
The biggest difference in terms of mechanics is the shift in emphasis towards modularity instead of multi-classing. In 3.5 if you have a character concept that doesn't fit easily into a pre-existing class, you have to do some crazy multi-classing to get it worked in. Paizo's "archetypes" approach makes that kind of thing a lot easier, because it lets you swap out class features in a standard base class in order to get something different. Not more -- just different. I used to spend ages working out how to qualify for weird prestige classes in order to get a character which matched the picture in my head reasonably well. Now I apply an archetype to one of the base classes, and never bother taking any other class. It's great.
The rules have to be open as far as I am aware because WotC unusually made the Open Game Licence where they open sourced their base rules, and so derivative works need to follow suit.
I'd point out that Paizo has continued putting the OGL on all their new stuff, too. It applies to all of their hardcovers (though large parts of the campaign setting book are exempt), and to portions of the adventure paths as well. For example, take a look at the Sanguine Ooze Swarm. It's a monster whose stats never appeared in any of the books; it put in an appearance in "The Haunting of Harrowstone", but it's under OGL, so it's up for free.
... [my group doesn't] see the point in Pathinder, as they have all the 3.5 books, so why rebuy them with another company?
That's exactly my point. They don't have to buy the books; the mechanics are all there online, free for use. Bring a laptop -- or better yet a tablet -- and you've got the entire rule set right there ready for use.
... Paizo SHOULD put more work in to their adventures, as they already put minimal work in to the ruleset and made a bunch of cash from it.
Dude, they put out a new adventure every month, like clockwork, as part of their Adventure Paths. Each adventure path comes out once per month for six months, then they start a new one. There are 9 finished adventure paths to date (54 total adventures), and they're two books into the tenth. The adventures themselves run 64 pages, but the actual publication generally clocks in about 92-100 pages overall once you add in new monsters, articles giving background information on the adventure setting, the monthly fiction, and the rather nice artwork. And that total doesn't even count the other adventures in their "modules" series, which are stand-alone adventures rather than part of a larger story arc. There are 50 of those so far.
Like I said -- they make their money mostly off the adventures. They're happy if you buy the books, of course, but what they REALLY want you to do is sign up for a monthly subscription to their adventure paths. That's where they make their real money. If you want a home brew game, or whatever, then you never have to pay them a dime.