For its time, it was the absolute pinnacle of what an action film could be. An action film that still has a fantastic story, a brilliant cast, great acting, some top notch dialogue (but that's a staple of any Tarantino flick), action scenes that actually use choreography and you can see what's going on in them, and do that all with an R rating. I'm still hard pressed to find many films from Hollywood that have done that since, and even then, I can't really say that they're better than Kill Bill.
I love the contrast of the two films as well. Vol. 1 establishes the conflict, shows that the main character is a badass with some deadly skills, and delivers on all the insane action. Vol. 2, however, is where her skills are put to the test against those who can be considered her equals or use unconventional methods, as well as showing where a lot of her skills came from, and have her question whether revenge is worth it. Instead of overly long fights, they're almost all short and sweet. The winner is mostly the one who has the mental edge or the ace up their sleeve. Where in the first film, it's simply go all out and kill anyone in the way. Vol. 2 has her nearly get killed a couple times trying this method.
All in all, Kill Bill is the Tarantino project I have watched the most (though it's not necessarily my favorite). It was another big hit for him from the fans and the critics, and he hasn't strayed from that (A lot of people hate Death Proof, but I think that it was exactly what it says on the tin: a Grindhouse movie as directed by Tarantino. He did the genre justice and did something not very Hollywood).