My group has played for many years and we mostly enjoy combat. Consequently, we focus a lot on the rules related to combat. Most of our characters and baddies are min/maxed to at least some extent (some players have more a taste for this than others). Here are a few of the house rules we've come up with:
1.) Max out hit points at every level. This goes for monsters as well. Combats last longer (which we like), and it's much easier to compute monster hit points.
2.) No death from massive damage, or kill threats (both presented as options in the DMG which we used for awhile). Basically, once you get to the higher levels, it's pretty easy for your spellcaster to do ridiculous amounts of damage, or your figher to have enough attacks that he kill-threats too often. We had a rather anti-climactic ending to an adventure when the fire-immune, cold-vulnerable, final boss died from a single Empowered Cone of Cold.
3.) Skill Focus gives +3 (a common house rule, it seems)
4.) No Psionic Combat. We gave Psions their choice of bonus feats at the same levels as a wizard, and Psychic Warriors bonus powers known. After a few tries, we basically felt the existing system was too complicated, cumbersome, and rarely-applicable to be used.
5.) Completely revised the Ranger class to make it more desirable at higher levels, and less powerful at lower levels. That would make for a much larger post, so suffice to say the overhaul was drastic yet well-recieved.