I agree. It's a very "talky" play with tons of exposition. Without any stage business to distract, it comes across as quite dry and almost stilted. Most of the actors were very good, although I thought Burstyn seemed less comfortable with the format, and seemed more obviously to be reading than the others.
Interesting, however, to compare with the movie, in which Bette Davis' character was considerably expanded. In the play, she's very much a secondary character.