Well, Pete - I've been meaning to get some Radiohead for several years now (after hearing some of my bro's
Kid A while driving down to the Alps), had been recommended by Lorsummit to try
Amnesiac, but after reading this and the comments following decided to try OKC when I saw it in the HMV sale the other week. I have to say it sounded pretty good at first hearing, and is definitely growing on me with repeated listening...which IME is a pretty good test of good music. BTW do you have the sheet music or did you work out all those harmonic progressions by ear?
The two tracks that really stood out on the first runthough were "Exit..." and "Climbing up the Walls", with "Electioneering" pretty catchy in an American sort of way. I'm with you though "Let Down" is pretty tedious (and I don't find much of interest in "No Surprises" either). When "Lucky" came on I thought that I'd somehow slipped into a parallel universe and it was playing a Pink Floyd track that got cut out of "Wish you were here..." (that subdominant modulation at the start of the refrain in Lucky is pure Floyd), and "The Tourist" has the same kind of resonances. As that is probably my favourite non-classical album, that is A Good Thing in my book, although I understand that this is considered the mark of the devil by several here

. I also thought "Paranoid Android" and "Climbing..." were also quite redolent of Portishead's Dummy - which is another of my all-time favourites.
I have a few things to add, mainly about "Exit...". I agree with some of what you've got there, but I think you've missed the two main features that make it so effective - the melody and the rhythm. As you say it's a really downbeat song, which is put over by each phrase being basically a falling fifth, starting each time on a resigned high and then dying from there - with the one exception of the line rising to, and that (orgasmic) suspension in harmonic progression in the bar preceding, the triumphant cry of "now we are one". But the really disconcerting thing is the rhythm - the insistent guitar chords throughout on the off-beat, with only a low bass note on the first beat of each bar, and the meandering vocal line unsure whether it's on or off the beat. The absence of anything on the main beats confuses the ear into thinking that the guitar is actually playing on the beat and the vocal line is even more disturbed, continually anticipating the beat. This is further compounded as the vocal line only actually sits unambiguously on the main strong beat right at the beginning of the phrase, moving more to the off- and up-beat as the phrase progresses. Things are disturbed even more as the guitar adds some subtle decorations, further emphasising the off-beat quavers, at the end of the second 8-bar phrase ("before all hell breaks loose"). But then it really gets going in the next section as the time signatures get a real shake up. Assuming it's been in a regular 4/4 until now (two phrases of 8 bars, with a 6 bar intro...which was a bit unsettling in its own right - there were a couple of bars where it felt something should be happening but we're not quite sure what), we get what sounds like a 5-bar phrase consisting of 1 bar of 3/2, one of 4/4, then two of 5/4 and one of 4/4 - reflecting the panic in the words "breathe ...don't lose your nerve" (and we've also moved into the dominant key here to reflect the contrast). That same pattern is then repeated, with an extra bar of 4/4 on the end to keep things out of balance - before we're back to the original phrase, with the words back to the original mood of desperate trepidation.
I'm not at all sure I agree with you about the chord at "laugh" - didn't seem to be anything shocking about it to me at all. Haven't we just moved out of the minor tonic key into its relative major for this verse, with its mocking-cum-triumphant words? It seemed to sit quite naturally within a chord progression leading through it (I had to crosscheck againt your timings to find anything of note there). The one point where I think the harmony is not pretty much dead obvious is at 3:10, where it turns from the expected to leads up to "now we are one". The other thing about the confidence of this section is that the rhythmic ambiguity has now all gone - the guitar chords may be still there in the background but are for the moment completely drowned out by the band and vocal line all firmly on the beat. When that dies down, and the words move on to "we hope that you choke", we get back to echoes of the original line, and key, and we hear the off-beat guitar back again. As with all good recapitulations though it's coloured by what happened earlier....listen for that triumphant cry of "now" still in the background at 4:00-4:07. I don't normally pay too much attention to the words of rock/pop songs (as they normally say so little), but this is a case where the music and the words really do form a pretty indivisible whole.
Re "Climbing...", did you notice that those two extra bars in the verses get balanced up by a foreshortened final phrase (only 6-bars) before the instrumental section, actually giving a 'conventional' 48-bar length for the vocal?