As a playwright, I'm hurt and yet fascinated and sometimes secretly thrilled when people HATE my plays. To draw hate, or even lasting irritation, from somebody is an artistic accomplishment. Of course, it's not an accomplishment anybody likes claiming, but it's still impressive.
That Isherwood is so annoyed by Adam Rapp that he feels he has to avoid his work is really saying something. Rapp has moved his mind. Unfortunately for Isherwood, he's moved his mind in a way that he thinks is negative or maybe trivial. But I don't think Isherwood can really think Rapp is trivial if he's SO bent out of shape that he has to stop watching his plays!
Presumably, Isherwood is paid to review. Probably he doesn't even *have to pay* to see Rapp's plays. So he sees a bad play. So what? Sometimes work is boring. Why take it so personally?
As a playwright, even though I can get really hurt if somebody dislikes something, I still think, "You know, it's just a play. If you don't like it, that's okay. THERE ARE ALWAYS MORE PLAYS."
Also, bad reviews are a part of playwriting. I'm not put off by Isherwood's bad reviews. If you're a playwright, you have to accept and expect that people will totally and maybe irrationally *hate* what you've done. It may take three months to write a play, but it will always only take one second for someone to say, "That sucks." But that's the way it is.
I would only add one thought: I've been a little dismissive of the outrage on Rapp's behalf, from fans who wish Isherwood had done this ages ago and/or from sensitive theater folks who are hurt on any playwright's behalf. I do think that Rapp can stand it, and his career has shown few signs of being slowed by Isherwood's disdain. But if I do a thought experiment and imagine a playwright I admire, like, say, Conor McPherson, being subjected to the longstanding scorn of a powerful critic I don't admire (as I do Isherwood, in fact), I'm willing to admit that my view—roughly stated, that Isherwood shouldn't lose his nerve and retreat from the field—might be different.