The most wonderful thing about this movie is that everyone in it is clearly having a blast. It's rare that you get such a strong sense that everyone on the screen is really enjoying themselves, but it's absolutely infectious. The second thing that I adored about the film was its untrammeled and unabashed celebration of sexuality in its many forms. You don't often get to see 40something year old women being sexual on screen, and man is it great. The wonderful thing about it is that it's not sex that's meant to titillate the viewer - it's performative, sure, but it's also wonderfully selfish - it's that I don't care if you find me hot because I KNOW I'm hot and I'm relishing every moment of it type of eroticism that you so rarely get to see, and oh how very refreshing it is. Especially because it's not the bitter, I'm hot in spite of your judgemental attitude kind of thing, it's the hey life is a party and you can stand there and be grumpy or you can come join in but either way ima have a good time type of feel. And it's not just about celebrating female sexuality, which tends to have a kind of exclusive and vaguely jaded feel, it's about everyone. Whether you're male or female, 20 or 50, you can get in on it too.
I think, also, a big part of the charm of the film is that it's somewhat rough around the edges. None of the big stars in the movie are particularly impressive singers, nor are they especially amazing dancers, but that's what makes it so open and fun and free.
The ABBA songs (and if you can't an enjoy an ABBA song, then we probably can't be friends) are skillfully integrated into the plot, giving them a kind of ridiculous poignancy that makes you laugh happily instead of groan. The plot is a bit melodramatic, sure, but it never really insists on being taken seriously, or gets in the way of the party.
Goddamnit, it's a great movie. It's the very epitome of entertainment. And that's all there is to it.