In what ways is "Educating Rita" by Willy Russell effective as a play?

'Educating Rita" is dramatically effective in many ways. It is also recognised as an exceptional play; it was voted "Best comedy of the year" when performed by the Royal Shakespeare Company in 1980, and just three years after release, it had become the fourth most popular play in Britain. There are obviously factors which make it so effective, and I intend to explore these factors. Throughout the play, there are only two characters: this is known …

…Just think if I'd let you see it when you first came here. Rita: I know. . . I wouldn't have understood it, Frank. Frank: You would have thrown it across the room and dismissed it as a piece of shit, wouldn't you? Rita: I know. . . But I couldn't have understood it then, Frank, because I wouldn't have been able to recognise and understand the allusions. All these points make 'Educating Rita' dramatically effective as a play.