It was not that i was not open to 3rd parties. It was just the first book i opened:
Poor quality. Obviosly so unbalanced that it didn´t seem fun in play.
Coming from 2nd edition you were used to blatand imbalances. But most funnily, all those imbalances balanced out. Mages were great, but a level 1 mage firing off magic missiles or a level 1 fighter standing next to you and you did not a lot during the fight. So team work was encouraged, even if mechanics of the fighter did not have anything to support, but the DM was usually playing monsters as smart as they were.
you also could not directly compare different classes, as they earned xp on different rates and also leveled up differently.
Of course a bard was a weak class compared to the mage, but he could protect itself in battle, could do something after his spells were used, and last but not least, he was usually some levels ahead of everyone, so he was not that far behind on the courve as it looked on paper.
With 3rd edition, putting everything on the same scale made everything comparable. If you then look at the bard... he was the only class in the transition from ADnD to 3rd edition, that LOST a spell (which could be compensated by high stats), he LOST his level advantage.
The fighter though looking great on paper LOST his great save progression and fast advancement and his special extra attacks per turn.
The wizard only lost one thing: the chance that his spells were interrupted.
Once again drifting away... what I wanted to say: when everything is on the same scale, yoiu start to compare. And IF classes that should fill the same role are obviously differently powerful, then you have trobles. And if the first book you look into and see (prestige) classes with abilities that are ridulously more powerful than anything else in those books, then you don´t want to have that in your game.
I am a little bit opposed to pathfinder, as it is too fiddly for my tastes. But instead of saying: lets take everything down to PHB 1 power levels they went out and said: Hey, we have those ridiculously powerful choices in 3.5, lets try to make a new PHB with classes that at least can compete. And lets make a lot of options to those classes, that a wide array of subclasses are covered.
This is what makes it so successful, i believe:
a resemblance of balance when containing yourself only to the Pathfinder PHB, and enough choices to make this containment not feel too narrow.