The Guardian says I'm not alone. (see previous)
I'd never seen Chekhov, except as rearranged by The Wooster Group a few years ago, but I've read him, and I understood the context. The Donmar Vanya was a disaster, and Twelfth Night was deeply off balance. The Guardian says it has something to do with the scale of the theater, and that makes sense. The Donmar Warehouse itself apparently is much smaller and what seemed overwrought at BAM would have come off differently in a more intimate space. Simon Russell Beale's characters, Malvolio and Vanya might have been the terrifying messes they were supposed to be if he was slobbering all over rather than beneath you.
All that said, I think the problem was more involved. As I was watching the plays I wanted to blame film and television for what I was seeing. Actors now are often very good at inhabiting their characters, are very skillful at 'being' them, but not as good at describing them as works of art. Art means artifice; it's an artificial construction, but it is an artificial construction as poetry. Chekhov's play is about a group of mediocre people; if you like Chekhov you'd call it a brilliant description of their mediocrity. But the actors had more respect for their characters than they had for the play, and they brought it down to their level. If Mendes had done a better job, had circumscribed their roles, controlled their gestures as a film editor cuts and pastes a scene, then the naturalism would have had more direction, and the artificiality would have been more precise. I got the impression that acting has become a memetic skill and not an art. The recreation of human character you see in almost every TV ad is amazing, but there's no art involved, just a brilliant imitation of the banality of life, and that's not what the theater is about. That kind of acting however is perfect for film; an actor's performance is cut apart and put back together ruthlessly after the fact. In the theater the actor must do it himself, and this skill is being lost. Young actors do not understand the art of theatrical acting.
I've heard there have been complaints about Russell Beale's weight. That has nothing to do with it, except as an adjunct to another aspect of the actors love for their characters, 'as they are'. I've always liked British actors for being blank slates and even empty suits off stage. I imagine Olivier as boring. Kenneth Branagh or Ralph Fiennes seem almost non persons outside their characters. Jeremy Irons, who ended up accidently holding the door for me while he was trying to sneak in unnoticed after the play, has always struck me as a perverse sort of Zelig. American actors on the other hand are generally one character, their own, or at least what the one character they imagine themselves to be, think Stallone or John Wayne. Charles Laughton professed an awe of the 'naturalness' of Gary Cooper. Why not? He wasn't an actor; he played himself. Maybe it's not film so much as the influence of the Actors Studio and method acting that's the problem: that American democratic ideal that whatever anybody is, is fine, that it all comes down to the individual and the self. It's interesting that people have said Beale is less the next Olivier than the first Simon Russell Beale, interesting that the actors in both plays were playing versions of themselves, playing to type in a way that I didn't expect.
But Russell Beale was impressive, physically and intellectually a stage actor; he presented his characters, not merely putting them forth for public view. His naturalism is an artist's naturalism, but in Twelfth Night his Malvolio was so overpoweringly tragic that it threw everything else out of whack. And again this goes back to Mendes. The orchestra had a soloist in an ensemble and no conductor.
Eveything I've said here also applies in varying degrees to Medea. Fiona Shaw is as much of a theater actor as artist as Russell Beale. Her performance was something to revisit and dissect. I can't say I liked or agreed with all of her decisions, but they were all made seriously, especially the moments of cruel humor, with a thought to the results as theater. The lack of such awareness in the actors and the direction around Simon Russell Beale holds true as well in the production surrounding Fiona Shaw.