I suppose accusations of "style over substance" have been bandied about at these moviemakers plenty, but hopefully the point I'm making here is slightly different.
Somebody wiser than me, I forget who, assessed the Matrix movies as a collection of all the things the Wachowski brothers find cool: black leather, sunglasses, slow-motion camera moves, Hong Kong action, "wire-fu", etc. Sure, there's a story holding everything together, but in many ways it's just an excuse to bring these other elements together. Kill Bill amplifies this, dispensing with any story complexity or character motivation complexity: The Bride is going to kill a bunch of people for revenge. End of story.
Even The Matrix Reloaded's much vaunted "subtle references" sort of stuff comes across as just cool. It doesn't seem to serve the story or its themes; it's just "wouldn't it be cool if this character had an ancient greek name? wouldn't it be cool if Agent Smith's license plate was a reference to a biblical passage about a Smith?" Picture Keanu Reeves hearing these revelations and going "Whoa, deep."
Or maybe "Whoa, cool."
Cool. Is cool an aesthetic itself? It is on first glance. "Our aesthetic for this movie is things that are cool." But I don't think that really works. You can try to form an aesthetic from anything: "our aesthetic for this movie is things that are rainbow-colored", but you're creating what I might call a style, not an aesthetic.
The problem with Kill Bill and The Matrix, in some sense, is the classic American "too much of a good thing". Cool elements are great, in reasonable doses. But a movie full of things just because those things are cool doesn't cohere into anything other than a lot of "cool" things, and the welcome wears out.
Consider, for instance, "wire fu". Kill Bill has only a few small instances of it (at least that are blatant), whereas Matrix does it to excess. In the case of the Matrix, there is a clever fictional setting that allows this impossibly superhuman behavoir to be justified, to be "real" in that world; but, even with the "clever" virtual reality revelation of the first movie, I still tend to see the whole setting as having been constructed to explain the cool action they wanted to put on screen--not the other way around. The superhuman actions does serve to demonstrate the nature of the Matrix and of these particular people participating in it, but it doesn't serve any particular aesthetic goal (that I can see) beyond the "it'll be cool" faux-aesthetic.
And in terms of "too much of a good thing", you have the "house/stairs" fight in Matrix Reloaded, which is just all non-stop wire work, and it, at the time, seemed overly long, tedious, and now in hindsight, not particularly memorable. (Maybe you remember certain moments from that scene with delicious joy. I don't.) It doesn't serve any real story purpose--Neo is fighting a delaying action so the other guys can escape so he can get lost and then fly in and fly off with them at the last minute... (why not fly off with them right off the bat?)
Compare this to the wire work in Crouching Tiger, Hidden Dragon--a movie which I am critical about the pacing, but not the aesthetics. CTHD's fights are distinctive and memorable. The "standing on the narrow limbs of branches" fight is perhaps the standout, especially in terms of cool. The impression is not "hey, wouldn't it be cool if we had these people standing on these branches that shouldn't possibly support their weight"; instead, it's the sense of magic of the movie's mythologized world. The description that I just wrote in quotes doesn't actually even sound all that cool. Instead, it's the way that moment connects with the overall aesthetics of the movie, and the story--the passivity of Chow Yun-Fat's character, the sense of effortlessness the moment provides to the characters.
Does "dancing on the trees" serve the plot? Not as such. It's included in the movie because it's a "cool" moment. But the coolness of the moment derives from and/or serves the overall aesthetic chosen for the scene (and the movie). It's cool in service of a further aesthetic goal, or cool because it meets an aesthetic goal. It's not just cool.