This month marks the one-year anniversary of my Savage Worlds fantasy sandbox, Autumn Frontiers. We're about 14 sessions in, which averages out to about one session per month—not too bad, but a little less frequently than I would have liked. Oh well, we're all adults with busy lives, so I'm not gonna turn up my nose at 14 substantive sessions in a year. And did I mention that this is the longest-running game I've ever GMed?
Anyway, we're using Savage Worlds, and over the last year we've tinkered mightily with that system. Most of our modifications have been designed to speed up an already fast ruleset. That's one of my weaknesses as a GM—no system will ever be fast enough for me, because I live in mortal fear of boring my players with drawn-out, grinding combats. So anything that speeds things along is paramount at my table.
The first thing we did was eject the playing-card initiative system in favor of a single d6 roll per side (one for the players' party, one for the GM's monsters). This also necessitated tweaking all of the various edges that reference initiative or being dealth the Joker, etc. Spending a benny can still win the players initiative if they so choose, however. I know the playing-card initiative system is a hallmark of Savage Worlds, but to us it just introduced 52 extra fiddly bits to our already crowded tabletop. Out it went.
We've also ignored a lot of the combat maneuvers (disarm, called shot, etc) as well as most of the edges that don't show up on character sheets. When I stat out monsters, I prefer to express their threat in terms of hard numbers rather than edges (which, like feats, are difficult for me to remember during combat).
We kept the skill list, but we only use about 6 skills regularly, the rest being relegated to specific situations or characters.
Really, what's kept us most excited about Savage Worlds has been the innovative resolution mechanic: Target Number 4, which you can attempt on a variety of polyhedral dice based on your relevant skill. But you're always trying for a 4, mostly. And any dice that rolls its maximum explodes, allowing you to roll it again and add it to the previous number. This can result in some hideously high damage rolls, both for the players and the monsters they encounter, and that's kept things very interesting out in the wilderness. Anything that rolls dice to attack you can, conceivably, drop you with one attack. We love it!
In retrospect, the path we've charted with this game has a lot in common with UncleBear's "Old School Anything" concept—just strip out all the extemporaneous stuff from your game, look at what's left, and run a game with it.