I think the guts of most RPGs can be disassembled into three mechanical categories:
1) Mechanical playing pieces (either buildable stuff like character generation, vehicle design schemes, etc or readymade items like spells and equipment lists).
3) Stuff you do that isn't combat.
Some games use a so-called 'unified mechanic' that attempts to squash items 2 and 3 together. Most of the time this claim is overrated, as you quickly find that the unified mechanic is highly modified with special cases and charts and crap once Fightey Time starts. Relatively few games use the exact same mechanics for a barbwire cage match and a tea party with the Queen. S. John Ross's Risus is about the only game I personally play that does that.
Not that I wouldn't try another system that works that way, though I am wary of such designs. A major pitfall with many unified mechanic games I've seen is that they tend to look like a perfectly decent game that would be good if only they had remembered to put in the chapter on combat. Somehow Risus avoids that sensation of playing a stripped down game. But maybe because I'm confident that the designer isn't a hippie trying to make a point about the unhealthy ubiquity of combat mechanics in RPGs. He's still pretty hippie-ish, but Ross doesn't have an anti-combat design agenda.
In the case of games that separate combat and non-combat resolution, pretty much every single time the combat section is a lot more interesting, both in terms of fiddly mechanical parts you can play with and actual nifty cool results in play. Off the top of my head I can recall only two subsystems that approach the same level of awesome as nearly any game's combat system: car chases in Savage Worlds and seduction in James Bond 007.
So here's my challenge to game designers everywhere: more cool rules for car chases and scoring with hotties, please! If your setting doesn't have cars, chases from horseback or spaceship cockpit will do nicely. And if your setting doesn't have sexy people, please go back to the drawing board and design a new game. Elfwood exists for a reason, folks.
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