Of course, the OGL, the Open Game License, is unrevokeable, and therefore safe. It also fails to resonate with customers the way the d20 logo did in its heyday, but then again, Paizo's customer base is probably sufficiently informed that they don't care.
I've adopted my own version of it for completely different reasons; I want to distance my game mentally from Dungeons & Dragons, which it only superficially resembles, in some ways. It uses a D&D baseline, but makes some significant changes.
I've been tinkering with houserules for a long time, and while its probably presumptuous of me to say I'm now done tinkering with houserules, I think my tinkering has reached a state where it's mostly done. What changes I make now are very, very minor.
Here's what I've done, in a nutshell:
1) I had a hard choice to make in terms of which rule system to adopt. Four candidates stuck out to me, all of them from the OGL movement: Pathfinder, Trailblazer, as a bit of a dark horse, d20 Modern + d20 Past, and of course Dungeons & Dragons 3.5 Edition. While I like a lot of elements of all of them, three of the four didn't completely make the cut, and here's why.
a) Pathfinder upped the power level in ways that I didn't like. This made it incompatible balance-wise with races and classes beyond the core D&D ones. I'm a bit of an exoto-phile; why would I be interested in playing a dwarf fighter when I can play a shifter scout, or whatever? Anyway, because this made supplemental 3.5 races and classes incompatible, it effectively cut off a lot of options that I wanted access to. Pathfinder has since released 6 new classes, at least in Beta format, but none of the six are really ones that I'm thrilled with from a setting point of view.
b) The same thing is true of Trailblazer, although of course Trailblazer also gives you some guidance in modifying the classes and races to make them all balanced with each other. At the end of the day, though, I realized that was going to be a lot of work. Both of these two rulesets failed to make the cut for this reason. However, that shouldn't be seen as a condemnation of any kind of either ruleset either; it just that I had a very specific statement of work that I needed my OGL-derived game to meet, and both Pathfinder and Trailblazer focused on some slightly different things. That's not to say that neither of them are useful to me, though... I've adopted Pathfinder's slightly modified, improved and simplified skill system and combat maneuvers rules, and Trailblazer's action points rules and combat reactions got integrated almost as.
c) I really liked a lot of the conceits of d20 Modern; it's like D&D except without the proscriptive class nature (classes are more descriptive and are clearly just bundles of mechanics, not role-protection), with a lower inherent magic level (you can't take any spellcasting class levels until you've already got at least three levels under your belt... and even then, the spellcasting classes only have ten levels) and a few other things. What made this rules option ultimately fail for me were two things: the haphazard nature of the modern SRD (MSRD) which includes d20 Modern, d20 Future, Urban Arcana, but which bizarrely never got around to incorporating the d20 Past rules that I considered essential, and the fact that I had other options just as good if not better for limiting magic and introducing chargen flexibility. This is still a close second for me, though.
d) Dungeons & Dragons 3.5 Edition therefore became the winner. The fact that I can use classes from all kinds of sources gives me the flexibility I want, and I can also take some other clever steps to reduce magic and flyaway power levels. As I said, it's not completely unchanged; I swiped a few other rules from Pathfinder and Trailblazer both. But I'm able to get what I want with the minimum level of houseruling using this rule system.
e) As an aside, I should mention that I never really considered a non-3.5 OGL option. 4e fails for me because I have no desire to go buy a bazillion new rulebooks to recreate what I've already got for 3.5. OSR-themed games fail for me because when the source material that they are mimicking was current, I was unsatisfied and unhappy with it. Prior to the release of 3e, I was almost an anti-D&D roleplayer because I had been so disillusioned with earlier edition AD&D and BD&D in particular. So honestly, I don't really know much about what's going on with games like Labyrinth Lord or Sword & Wizardry, nor am I particularly interested in finding out.
The 3.5 ruleset is also such a gigantic buffet of attractive options, that I never really considered another system either; although if I were to, I'd probably have a look at Savage Worlds.
2) The E6 top-hat was an essential ingredient for me. E6 postulates that looking at fantasy fiction, its hard to believe that most literary characters are really very high level, or that the advancement of characters as they gain experience and skill is so exponential as in D&D. It also is based on the simple framework that as characters progress in level, the game actually changes genre; going from a more sword & sorcery type game to a wuxia fanasy superheroes game somewhat unexpectedly (well, unless you've been on the curve before, of course.) E6, then, says that 6th level is where your character tops off. You never get any more levels after 6th level.
You can, however, continue incremental advancement; every 5,000 additional experience points a character gains, he or she also gains a feat. There's a few more rules clarifying the odd corner cases that occur here, but that's basically it in a nutshell. Higher level magic can be accessed at the GM's discretion by using the Incantations rules, which were printed in both Urban Arcana and Unearthed Arcana and are therefore open content.
3) Action Points become an integral part of the game. I've actually expanded the use of them somewhat, giving them the option of working not a little unlike 4e's healing surges, and also rather than resetting each level, I give players fewer of them and have them reset each session. One of these days I'd like to buy a bunch of Campaign Coins and hand them out as Action Point counters, but in the meantime, pennies, Skittles, or just about anything else will do the trick easily enough.
4) The Defense Bonus is one that I think is integral to the game actually working correctly. Why is it that attack bonuses go up as you improve, but not your armor class? Why is that equipment dependent? And what if I want a more swashbuckling type of game rather than one where fighters clunk around in heavy armor?
Well, Defense Bonus (from Unearthed Arcana) is the solution.
5) Chase rules. I like action movies. To me, fantasy roleplaying games should resemble literary fantasy fiction and action movies. A staple of action movies is chase scenes. Oddly enough, d20 doesn't have any native chase scene rules. I've got a few options available, and I'm settling on a paraphrasing of Wolfgang Baur's chase rules, which appeared in Five Fingers (and maybe somewhere else, too.) Why those? Because they're the most simple. I could tell you how to run a chase scene with these rules in a modest-sized blog post (in fact, sometime later I will). In a lot of ways they resemble complex skill checks from Unearthed Arcana which in turn were ancestral to 4e's skill challenges... but hey, that's an easy ruleset to adapt.
6) Not exactly a rule, but what I most want to do is change the D&D paradigm. Do I have enough houserules that I can honestly say that I'm not even playing D&D anymore and am instead playing some other game that uses the basic D&D rules as a baseline but then goes somewhere else? I dunno. That doesn't matter. Changing the name of my game is more about changing the paradigm that I want at the table. I don't want a game of heroic good vs. evil. I don't want a game of dungeoneering for profit and XP (or for any other reason, for that matter.) I don't want a game where the players assume that they're supposed to win, no matter what they do.
I want darker games. I want shady characters engaged in shadowy intrigue and plots. I want an urban and wilderness environment that's more exciting than the prospect of conveniently placed ruins, or "dungeons." I want magic to be frightening and unnatural. I want players who play their characters. Who know that they could get swamped and killed if they insist on the wrong fight. I want horror, I want action, I want swashbuckling, and maybe even a little steampunk. I want a sword & sorcery weird tale, not a high fantasy epic.
Anyway, I'll blog some more in the next little bit with more detail about some of the houserule changes that I've settled on. I've also created the house rules tag, so all posts related to house rules present in my game can be easily found with one click.