I don't really go in for big house rules, especially since most of my Munchkin playing is done in demo settings. The biggest change I make is that I use minis on
the level board to track levels instead of using d10s.
I've also been known to give an extra +1 if somebody winds up with a treasure that's just uniquely fitting somehow - like a couple of years ago when the chainmail dealer got the Chainmail Bikini. He'd brought a chainmail bikini top with him to the table, and I offered him the extra +1 if he put it on. Sure enough, he did - strapping it on his head like a helmet. :)