As much as I like D&D and OSR-style d20-based gaming, my real RPG love is, and has always been the BRP/d100 family of games.
They are easy to grasp and have simple and consistent rules for most situations. There are also variants that will let you play high fantasy, dark fantasy, horror, modern, sci-fi – basically whatever you want.
What I like the most is that the characters, even if they’re veterans, are fragile. If you’re outnumbered or if you’re surprised in the wrong situation, you’re going to get hurt. And there are no potions of healing anywhere to be found (at least not in my games). This automatically leads to another style of play from the characters – more cautious, more thinking and planning before doing reckless stuff. There are no artificial levels, just different levels of expertise that you learn the hard way. And combat is deadly. One misstep or lucky critical hit will kill even a veteran character.
That said, doing D&D-style subterranean dungeon romps with d100 rules is nearly impossible. The PCs will surely perish before they even reach level 2 of the dungeon. As a GM, this naturally encourages me to play other types of adventures if we play d100-based games. And players that insist on solving problems the D&D way will die. Quickly.
Having been away from GM:ing d100 gaming for over a year and coming back, I realize (again) that d100 games have about all that I want from an RPG. Also, the cross-compatibility of these games lets me take a Call of Cthulhu monster and chuck it into my OpenQuest or RuneQuest game with almost no hassle.
I think I’m going to stay in the d100 field a bit longer this time around, as we always seem to get dragged back to D&D by some unseen force…
If you’re interested, here’s a link to the recap of our latest Call of Cthulhu game session