FWIW, domino, I had almost the exact same reaction as yours the first time I saw it. I loved the concept and appreciated a lot of the ideas, but I felt like the novelty and insight was starting to wind down by roughly the 45 minute mark. Then, for whatever reason, I felt compelled to watch it again the very next day, and it completely clicked. I think it's important to the film's power that it should feel sort of like the premise to a (really really weird) lost episode of
The Twilight Zone stretched far beyond the point of logical conclusion. Oshima sets up for himself an ingenious scenario with which to allegorically portray the fundamental sort of corruption underlying Japanese society: he gives us one figure to represent each element of that society (from religion, the military, and bureaucracy to the sort of bourgeois perversion of the doctor and the brutish ambition of the one guard who worked his way up the social ladder) and has them oversee an execution that's covertly loaded with issues of race, nationalism, economic inequality, etc. But rather than trying to encompass complete or implied arguments about those issues, Oshima instead draws out all the implications (or allows them to surface on their own) allowing the film and its ideas to become knottier and knottier. It races through concepts with such speed, it's actually forced to become something of a didactic piece (albeit still an extremely complex and thoughtful one, which doesn't always take what it has its mouthpieces say at face value) in the third act, as the dramatic structure and narrative start to slide away due to their inability to contain everything that gets drawn out. Now, I realize this is the kind of hyperbolic statement that could have the unwanted effect of making you resent this film even more, but it's probably the most fiercely politically engaged film I've ever seen (beating out even Godard and Marker). It's an absolutely ferocious statement that gets to me and into my head like no other - fortunately, my disc just came in the mail, so I can experience that ferocity all over again.
Shifting gears completely, there is of course also the possibility that the film just isn't for you. If the reenactment jokes (I presume you're referring specifically to the film's making light of R's crimes) did put you off - which is a reaction I completely respect - I wouldn't want to try and force that on you. It does make me wonder how you've managed to appreciate any films of the Japanese New Wave, as the juxtaposition of horrific, deviant, violent sexual action with absurd, sardonic humor is a pretty common trope (although I actually have difficulty with Imamura for apparently the same reason you dislike some of Oshima's films - as a side note, that difficulty I have, while sort of frustrating given the praise I've seen lavished upon Imamura on this board and elsewhere, is also kind of a relief to me, as I've long been somewhat fearful that a longstanding semi-morbid fascination with
Tosh.0 has desensitized me from that sort of humor, which I do often find objectionable on a basic level).
And I just saw Big Ben's comment:
But if R were innocent, the whole film would lose its meaning! Oshima is arguing that the death penalty is never justified. He couldn't have, as his main character, someone upon whom the operations of the justice system are that random and unreasonable. The reason the film is so devastating, for me anyway, is because Oshima presents us with the perpetrator of a truly heinous crime who rejects, on an existential level, his own culpability for that crime. It's not a presentation of the justice system as just faulty or corrupt. We come to see it as a cancer, growing naturally from the basest impulses of society at large, while simultaneously acting as a kind of a barbarous antibody (if I'm using that metaphor properly) destroying an exterior disease before it infects the system. In the end, it's not so much that the film doesn't want to call for an end to the death penalty, it's like it's genuinely impossible, so deeply ingrained is it into the way society works. And if, like R, you reject it in the most basic core of your being, you can't just return to the society that spawned it and live a happy life of ignorance. Becoming yourself a victim of the state - and accepting the guilt that the state places upon you whether you yourself feel it or not - is the only non-hypocritical course of action.