The only real house rule I implemented was a D% Wound table, ala' Hackmaster, but consulted every time someone is wounded (some exceptions apply, such as damage forms that don't allow a save or attack roll and can't be avoided, like magic missile). Since I brought it up as optional at first, and we playtested it a bit, and made some changes, no one in the group wants to play
unless we use "the table."
Which was pretty much a shock to me, since before I started DMing, our previous DM has some pretty crappy critical fumble rules. (If you rolled a "1" on anything, you failed, your turn was automatically over, and if it was
impossible to strike an ally, you did. (Like attacking around a 10' wide pillar to hit an ally with a dagger, or hurling your greatsword 50' to strike the mage in the back)
The best way to introduce a house rule it to discuss with your group
why you think the change was needed. I wanted a more gritty, realistic combat, not some "Ok, the great axe does 8 damage to me? Oh noes, 80 hp left..." My players agreed, and thus the table was drafted. We still make changes and addendum's to it every once and a while. (Example: Diehard prevents the Auto-unconsciousness, and Endurance provides +4 to all the saves against the table, making those two feats really attractive options) But over all no one complains that it is "unfair". (Though we take turns rolling the penalties, and one member of our group somehow manages to roll headshots for everyone else. He isn't cheating or anything, just pure dumb luck

)