I didn't quite get the difference in combat, but here's the three main changes:
-Combat damage no longer uses the stack
-Deathtouch is static ie. non-cumulative(no need to regenerate twice if a source has double-deathtouch)
-Lifelink is static-You can keep yourself alive with likelink, even if you have been declared lethal damage during combat. It is also non-cumulative.