I never include spoilers in a review (I'm always amazed that so many other reviewers do!) but I also never include the best lines (ie the ones that get the biggest laughs or strongest audience reaction). Is this good or bad from the readers' perspective?
Ooh. Interesting question. I don't like lines to be in a review of a new show, but if it's a revival and the book is reasonably well-known I guess it's OK to talk about the direction and delivery?
I struggle with this for comedy reviews. I tend to write something along the lines of "this is really really funny and you should go and see it because I imagine most people will find it really funny". But slightly better worded.
I think it's OK to give an example of a line that really worked (or didn't) in a review, as it gives the reader an idea of the type of humour and whether they'd enjoy the show. Brilliant shows have more than one great line, so you're not ruining the whole experience by citing one.
For me giving away a line is just as much a spoiler as revealing the plot - if the line is that good, it's better to be surprised and experience live in the theatre like everybody else.