I've been using the D20 system for the last two years, telling myself I should give it a try and that it would be easier, as Mongoose was releasing tons of material with that system. As everybody knows by now, I wasn't really satisfied with the rules and came back to a home version of the Chaosium BRP.
I use The Elric system (Chaosium version) as a base with some modifications, some homemade, some borrowed from other systems. The new edition of the Basic Role Playing was helpful too.
Some of the changes are concerning Hit Points: I use CON+SIZ for characters and main villains, and CON+SIZ/2 for goon type NPCs.
I changed weapon damage which I always found clumsy in BRP (like 1d8+1+1d4), opting for fixed STR bonus (à la OGL) and d6 weapon damage by weapon size (ie light, one handed, heavy... a bit like WHFRP). I use the new BRP rule for criticals by weapon type (Slashing/Crushing, Impaling...)
I use Hit locations but not hit points/Loc, adding extra dmg dices when hit in vital areas like head or chest. It makes combat more visual and allows to wear only some armor pieces, which is cool...
Initiative is DEX based as in most Chaosium games, with modifiers for wearing armor, heavy weapons or being wounded.
I also use the new BRP rule for Fate points, which allows to spend MPs to reroll dices, soak damage and so on.
It's not perfect, of course, but the game is much more fluid and vivid using these rules than it ever was with d20.