A couple of houserules

Regardless of which system I use, LL or 4e, I'll definately have a couple of house rules.

Labyrinth Lord:
I'm still not sure about variable damage expressions. After reading the 1974 edition I've come to like the idea of fixed 1d6 damage per weapon. Players could then choose their weapon depending on style and not efficiency considerations. It's also quick and easy.
What I really like is "Shields Shall be Splintered".
I'm also considering the "Dungeons is dangerous" rule as proposed by Jeff Rients:

Dungeons is Dangerous
Ending a session inside a dungeon requires a roll on the Triple Secret Random Dungeon Fate Chart of Very Probable Doom. Make sure you get out before the session ends!

D&D 4th edition:
I definately would want to track things like ammunition, thirst, and hunger.
Putting in Wandering Monsters is also a must (and extremely easy, look at the ideas of 4e Lead Designer Mike Mearls!).
I think I would cut down on xp per monster (to 50% or some such) and then introduce xp per treasure found, like back in the day.
"Dungeons is dangerous" would be used here, too ;)

These are the only ones I can think of at the moment, although I'm others will come to my mind another time.