Firstly, I think it is important, in examining these films, to do so as if they were one undivided whole. Tarantino has no presented this films in a temporal-linear fashion and one film without the other either lacks background or lacks resolution. I will be examining "Kill Bill" in its entirety through the lens of the portrayal of violence in mainstream cinema (which I believe Kill Bill fits under as it made $330,000,000 at the worldwide box office), and also slightly more specifically, the portrayal of women in violent roles in mainstream cinema.
Tarantino is a prolific (some might argue, excessive) user of pastiche in his films, and has himself said "I steal from everyone!" The plot of "Kill Bill" is based upon a 1973 Japanese revenge-drama entitled "Lady Snowblood" and the 'stealing' doesn't end there. Tarantino essentially creates of collage of genre tributes ranging from Spaghetti Westerns, to Japanese Samurai to Chinese Wuxia. He interlaces this with an appropriation of characters and quotations from other films.
It would be impossible to argue that "Kill Bill" is not a violent film, to the point perhaps of gratuity, and yet comparably violent films have barely had a modicum of the success that Kill Bill has enjoyed. I would argue it lies in Tarantino's very specific use of filmic techniques to either exaggerate the violence to the point of ridiculousness, desensitise the audience to the violence by limiting our sensory reaction or by gradually preparing us for a shocking act of violence.
The first technique, exaggeration of violence is perhaps best seen with the Tokyo restaurant fight scene where 'The Bride' takes on the The Crazy 88 and Gogo. The rather cheap looking effects, the blood spraying the roof and walls, and over-dramatised moaning all tend to create a scene that is beyond the constraints of reality, thus limiting the impact of the violence.
Secondly, Tarantino often removes one sensory aspect to decrease the impact of the violence playing out on the screen. This is seen in the very first scene when Bill shoots the bride in the head and the scene is in black-and-white, this serves a dual-purpose of paying homage to the spaghetti western genre of which Tarantino is so fond. We also see the same technique used again in the Tokyo restaurant when the lights are blacked and the scene is back-lit with a blue aura. While, every physical action is still visible the gruesome impact is far less.
The other technique Tarantino uses, is perhaps best described as gradual preparation. Sort of like stretching our violence-enduring muscles. This is seen with O-Ren's gruesome back story, which is animated and then leads into her decapitation of the Yakuza crime boss who dares mention her heritage.
While the industry is undoubtedly improving and becoming more 'female-lead' friendly, it's naive to say that there is equality in the cinema world. Placing a woman in a violence-heavy lead role, instantly alienates a large portion of the possible audience, men who instantly feel emasculated by a woman chopping men to bits, and women who are so suppressed by their society that they themselves perpetuate these cemented gender roles.
A number of cinematic scholars have described the woman's role in cinema as "passive" ("Visual Pleasure and Narrative Cinema" Laura Mulvey 1975) or while 'men act, women appear' ("Ways of Seeing" John Berger 1972), it is this aspect of mainstream cinema that makes "Kill Bill's" success all the more surprising and wonderful. The character of The Bride challenges every aspect of this male dominated world, while of course Tarantino isn't the first to do so, his is an important example for one reason.
While it could be argued another important example of a strong female lead is found in Ridley Scott's "Alien" there is a striking difference. "Kill Bill" is overflowing with female characters (lead and supporting) who are of different races (Caucasian, Black, Asian) who engage in violence and do so without playing second-fiddle to a man. The exception might be found in the character of Elle Driver who follows Bill's commands without falter, although one could argue their relationship is resemblant of a General/obedient-Lieutenant, rather than a suppressed woman.
It would be foolish to disregard the importance that this film played in helping continue society's lesson in cinematic equality and Tarantino's use of pastiche and genre-blending is undoubtedly intriguing. And if any one disagrees...