I think I enjoyed it more than you did,
PW I too was irritated by the constant televisual gimmicks with the boxes, though for me the point at which it grated the most was during Brangaene's warning, which was the first time Tristan and Isolde had been alone together against the "night" backdrop without the big pillar thing in the middle. The moment would have been far more effective with no camera change at all.
I was at the Barbican. It was sold out, the audience was very mixed in age and there wasn't too much audible snoring, though there was something going on next door in the Pit Theatre with a very loud rock beat which was slightly audible from the cinema - Act 1 caught its matinee and Act 3 caught its evening performance... As for the transmission itself, we briefly lost the signal twice during Act 1, and when it came back the picture and sound were out of synch (though it did eventually re-synch).
Robert Dean Smith was, to my mind, excellent. I almost returned my ticket after Heppner cancelled, but when RDS was announced, I considered him the best alternative they could possibly have come up with. I am flabbergasted that it was his Met debut. WHY have they not hired him before? Voigt was not quite as good; I don't think the role suits her as well as the Strauss roles she sings. I found her very credible in the second and third acts, but her intonation was off at the end. Also very much liked Matti Salminen and Eike Wilm Schulte (though whenever I see him I think of his Beckmesser!) I agree with you about Levine's Act 1 prelude (though it could have been worse - I heard Paolo Olmi conduct it the other night and if it had been any faster it would have been a waltz...)
I liked the production. I know there wasn't much of it, but I don't think Wagner necessarily needs a lot of visuals. I thought at least it should have ended against a dark background, but then again the last Tristan I saw was the Glyndebourne one, which did.
Did they really charge £30 a ticket in Brighton? I thought the UK standard price was £25. I got mine for £17.50 being a Barbican member.