This current campaign I am in is currently taking place in Faerun, and the DM opted to run 3.5 pure after reexamining his earlier position where both 3.5/PF would be allowed in. My argument was that given the same amount of skill, a PF build was mechanically more powerful than their direct 3.5 counterpart and that to run the two simultaneously would lead to serious imbalance, because PF has evolved/developed beyond what 3.5 could handle.
Not to say that PF < 3.5, because the predecessor had a lot of incredible exploits itself (EX: Loved Wildshape, hated Grappling), but that just as 3.5 bloated with expansions, so did PF's consistent growth entails power creep, plus options that were never possible in Core 3.5.
A veteran player who joined the campaign today requested to run a PF Fighter versus a 3.5 Fighter, and we had to shoot him down. The three extra feats by L20, Bravery, Weapon Proficiency, and Armor Expertise may not be game breaking in a world of magic, but tack on the archetypes, and heck, that PF Fighter DOES have a lot more potential than a 3.5 Fighter. One could argue that PF Fighter is as versatile/effective as a 3.5 Fighter that had some Gestalt features. Heck, with alternate race traits, one could optimize to that level of effectiveness. The 3.5 Rogue and Barbarian that would be running next to him would look pretty bleak.
I'm not looking to raise controversy, but ask opinions on where PF and 3.5 are no longer compatible; I'm pretty sure a final divergence could have been as early as CRB 1st Edition -where the first thing I noticed was all the sweet goodies plain progression classes like Fighter and Sorcerer received a lot of goodies to where MC/PRC was no longer so necessary, but when cool stuff like Archetypes, Alternate Racial Traits, and a lot of the background stuff was being published. One of the most attractives features at launch to me was the revamped selection of feats and likewise revised skills. Eight Skill points/Level as a 3.5 Rogue looks impressive until one has to deal with having to dump multiple points in very similar fields. Synergy mitigated this, but a lot of PF PCs have more accommodating skill allocation than 3.5.
On a counterpoint: under what conditions do y'all feel where a Pure 3.5 Adventurer can be just as effective as a Pathfinder and be able to run in either's mechanical setting?
False comparison. You must compare Core 3.5 to Core PF, or "3.5 + splat" to "PF with Advanced *, Ultimate *, setting, etc.".
It's simply absurd to compare Core 3.5 to "PF with all the splat" — but that's what you're doing when you bring in archetypes. Under the proper comparisons, a 3.5 character can easily win. To wit:
Core vs. Core:
1. 3.5 druid beats any PF class. (Wild shape and animal companions both worked very differently in 3.5.)
2. Certain martial builds were better in 3.5; for example, a 3.5 spiked chain tripper is superior in almost every way to his PF counterpart.
3. Bards had certain advantages in 3.5 due to the way song effect durations worked.
Splat vs. Splat:
1. PF simply doesn't have some of the game-breaking combos that 3.5 splat allowed.
2. Certain magical spells and effects (polymorph / shapechange is just one example) were way, way more open to crazy-power-level exploits in 3.5.
3. The far, far greater selection / diversity, and much lower level of standardization, of prestige classes and certain other kinds of content (items, spells, etc.) in 3.5 meant that some truly titanic builds were possible: nightsticks and Divine Metamagic is not a thing in PF, for example; nor is Vow of Poverty; nor is Mindrape / Love's Pain for unavoidable infinite-range ignores-all-defenses assassination; etc., etc.
Edit: In short, if you allow both all 3.5 material and all PF material, and someone builds a PF character and someone else builds a 3.5 character (both at high optimization levels), the PF character doesn't stand a ghost of a chance.
Edit 2: Of course, you could bring in 3pp stuff to help the PF guy out, while still sticking to WotC-only for the 3.5 side; but I am reasonably sure that even then, the PF guy gets stomped easily.
That all having been said, my ideal simple solution to "core 3.5 is what we want to run, but the lower-tier classes are kind of weak and that's no fun :(" is to use the Trailblazer rules. They fix what needs fixing, in straightforward ways (taking some great ideas from Pathfinder and adding a whole bunch of their own, rather ingenious, innovations).
(My ideal solution, period, is to design my own variant system. But I can't recommend this option to anyone else, because it's staggeringly effort-intensive.)
Thanks for the reply Makhno -should not have generalized on my end, but its hard to define exactly which books directly replace/analog their 3.5 predecessors.
I should have clarified, we aren't just running 3.5 Core, but a lot of the Wizards splat allowed. As in the Complete Series, Faerun, Eberron, MMs, UA + additional PB/DMG, to include Dragon Magazine allowed, 3.5 went a lot of places, as is PF. For example, had I gone Cleric, I would definitely have brought in the Exalted feats along with books like Libris Mortis. I think one of the few expressly excluded was Draconomicon.
I think they just use different routes of optimization. New Vet ended up going with a Warforged Fighter who opted down on CON for INT/CHA to not be low values. I kinda feel bad because I did as much research as I could. Uldra, Unseelie Fey (Vernal Touch), Wild Reaper (Variant Druid), and I'm debating on having a Dire Phynxkin from Dragon Magic and trying to compensate with Natural Bond as a feat-just proofreading that makes me feel like a bad person. Had the DM not stopped me, I would have taken another +0 template like Arctic thematically cuz the only limit was +1 LA -he didn't expect template stacking. Frostburn & the Dragon Magazines are what I pulled by plan from. Kinda munchy compared to his extra book being ECS. I really became entrenched in the PF character building process, and it followed me back to 3.5 where originally I was a happy-go-lucky Half-Elf Ranger.
PF I believe deliberately omitted a lot of the cheese that 3.5 made possible, but by continuously expanding, is enabling its own unique sets of exploits -the developers/playtesters are mindful of this, but the system inherently breeds optimization and a Munchkin, like Nature, finds a way. I think power creep comes with evolution and continued success/growth of the franchise and is a natural development.
I suppose I'm too easily enchanted by shinies, because PF just seemed so much friendlier to prospering by sticking to a class and tuning it to one's playstyle without having to go PRC using archetypes and racial adjustments. Maybe at entry-level its easier to kick ass, but I may just be making another ill-thought generalization
Vow of Poverty was a fun way to play a Monk, but a lot of the stuff just sounds foreign to me; thx for the enlightenment, I'm kinda scared to look them up for how they'll corrupt me further.
Vow of Powerty without epic skill use is almost worthless, and even then it can cripple martial classes, especially monk. It is fun but without spellcasting or manifesting abilities enemies with their own set of supernatural or magical abilities will start becoming really frustrating to deal with.
A 3.5 character 'done right' will wipe the floor with his PF counterpart, if all splatbooks are in. That's not news. It's not that PF doesn't have its own array of exploits and powerful options, it's just that the ceiling is considerably lower for a PF character while a lot of popular 3.5 builds eventually reach a point where they simply divide your game by zero. Now, if you're looking for something besides total game domination, PF indeed has more interesting options to fiddle around with (at the cost of increased game management issues).