Early games were focused on the very broad activities of dungeon and wilderness adventuring, and were neccessarily rooted in the need to reflect those kinds of things in their mechanics. But even today, many mainstream games still tend to be much less focused than something like Dogs or even Burning Wheel, not neccessarily to try to be all things to all people, but to allow those seeking some level of variety in week-to-week adventuring to at least dabble in different adventure formats; one session a dungeon crawl, the next a murder mystery, with another a courtly intrigue. A game like, say, RuneQuest (D&D, depending of which version you’re talking about, tends to be a bad example,) may not do dungeon crawls as well as D&D, mysteries as well as Gumshoe or intrigues as well as Reign, but it’ll do all of them functionally and passably well, which gives the world and story a sense of openness that something run in a very focused system may lack – and I think that’s something you need if you’re going to run a campaign for a year or two, that touches on many different themes and approaches.

This does apply to a certain extent to OSFRPG, which is intended to emulate a pretty specific style of game, or, more properly, a limited but not exactly narrow range of styles. As it develops I hope you’ll see more of where it’s intended to go and what it’s intended to do.

This is, in my limited experience, the key point. A narrowly focused game can do a better job of modeling a specific type of gameplay (e.g. diplomatic intrigue) better than a catch-all system like DND can. The issue is that, the more types of gameplay you try to cram into the game, the harder it’s going to be to get your system designed and tested properly as a part-time homebrew operation.

P.S. Bonus points if your “role playing” game system doesn’t force players to choose between skills that make sense for the character’s background and skills that determine their combat effectiveness. This isn’t to say that a good group of players won’t find a way to have fun with a party member who is pointedly ineffective in battle, but it’s a bit irritating that most systems make you choose.