Well, there's the rules. I "hacked" Old School Hack (along with a bit of Redbox Hack and DINO-PIRATES OF NINJA ISLAND Basic Game--all of which are variations on the same ruleset, basically) into something that I can use for my setting.
For the most part, I went with Kirin's version of the rules--that is, the Old School Hack version, but with Corey's class sheets--minus the ninja classes, which were too specific to his setting to really fit in mine. When I imported Kirin's Fighter and Thief classes back into the mix, I also discovered that Corey had re-used a bunch of talents from the various OSH classes and mixed them up. That meant that I also needed to "remix" some of the talent offerings a bit to make sure I didn't have any duplication (since there's a rule for taking cross-class talents already, I figgered duplication was a Bad Idea™.)
Other than that, where I differed most from Kirin's vision were in two ways--1) I didn't want to emulate any version of D&D (Redbox Hack curiously doesn't feel nearly as much like Redbox D&D as OSH does--which I'm sure was one of Kirin's design goals for modding it into OSH), and 2) I really dislike role-protection, and wasn't interested in following that route. I also didn't catch a few other latent "Old School" isms in the rules, like the way in which stats are rolled and assigned; I didn't follow suit. On accident at first, since like I said, I didn't notice the difference, but now that I've had them pointed out to me, I prefer the way I have it set up anyway.
The "hack" games are a curious artifact in some ways--while in many ways they are meant to be nostalgic and reminiscent of "old skool" D&D gaming, in many ways they're also very modern in the way that they operate--very rules-lite and easy to run (not that Redbox D&D wasn't also relatively pretty rules-lite, but it evolved into something that was not, of course.) Since I feel a bit less nostalgic than many gamers about my early D&D days--y'know, the days that drove me away from D&D and into (eventually) the arms of the Storyteller system with their pretentious approach to gaming--it's natural that what I'd like best about the Hack games are the more modern aspects and what I care quite a bit less about are the more nostalgic elements.
Also, I added a few new elements. They weren't exactly revolutionary, but they were new to the Hack family of games at least--the concept of Race that's separate from class, and the concept of Sanity. I treated Race as mostly a roleplaying hook, but each race has the ability to take a racial talent. This takes the place of a class talent if you choose to take it. You can never take another race's racial talent as a cross-class talent. And, of course, you have to choose to take it in place of one of your class talents, so it's not a freebie talent. And Sanity works very similarly to Health in many ways. It's perhaps a little bit more complicated when you get a "mental scar" but as someone who's had tons of fun in the past with Sanity episodes in Call of Cthulhu games, I think that the minor complicaton addes significantly to the game for those who like that kind of thing. If you don't--well, it's easy enough to ignore.
Only one or two of my talents were newly written by me; so I was more about reorganizing the existing game into something that fits my setting than I was in actually designing stuff. I don't really consider myself a game designer anyway, and writing mechanics is something that I don't particularly like doing or feel comfortable with much. But kitbashing existing mechanics into a new form--I'm all over that.