I've run a historical horror (ala Brotherhood of the Wolf) oneshot and asian fantasy. In both cases there weren't rules covering specific areas, but it was easy to take similar rules in D20/D&D and convert them over. I had to do this with early gunpowder weapons and feat trees for Shugenja class charaters.
I think the looseness of the ruleset opens players up to more creative tactical play. There's no AoO to worry about, and grappling is more streamlined. Since magical system is "generic" I had players coming up with some pretty fancy things rather than fire and forget a list of spells.
My players liked the greater amount of customization that they have in character creation. Players are no longer "forced" to take class feats they won't use or suffer from getting no new feats when they level up. They didn't like how damage took its toll, but you have to introduce them to conviction points and how they can be used to pull their fat out of the fire.
I liked that the system subtly encourages players to think encounters through rather than killing and levelling up. The GM holds the power for when players level up their characters, so their focus shifts from "when will I get enough XP?" to "I want to finish this adventure." Foes don't "disappear" when they hit 0 HP, they get incapacitated, and if you wait around you can interrogate them. This helps provide me with opportunities to have the captured NPCs give information that propels the plot forward.
I liked the minion rule and now I no longer dread all the time spent when players combat 2x or 3x the number of foes. I like the nature of damage - when you get hit and take damage there SHOULD be some consequences to your wounds.
I know that many of these things I have the ability to houserule into a D20/D&D game, but its so much easier when it's already written into the system. Rather than be thought of as a GM running a heavily houseruled game of D&D and the players some how feel "cheated" that I'm not playing it RAW, we're just playing True20 and having a fun time.
