It's too soon to really get into house ruling stuff... but I strongly suspect that I will use 2 house rules early on.
1. Slow Experience. Depending on how the xp system works, this may be as simple as cutting the xp for encounters in half, or it may be a complete rewrite of the xp system (very unlikely).
2. Colorful Critical Hits. This will largely depend on how the new system for hit points, total healing after a long rest, etc. works out. I like critical hits to break bones, sever limbs, knock out teeth and eyes, damage brains and so forth. But if 4e does a convincing job with its implementation of damage, I might have to abandon- or at least significantly revise- my Colorful Critical Hit system.