Fun, I say. ABBA songs are so gloriously orderly and deceptively simple. Some boys are driven to these manifestations of incipient drag-queenery by the Supremes and girl groups of their ilk, but I'll take a quartet of Swedes every time. And with the benefit of a few years' exposure to the machinations of the music industry, I now see the pleasure ABBA provides as that of a pop archetype. This does not mean that ABBA lived on some cultural or semiotic island.
Some songs are actually about the pop-music experience.
I find I am mightily charmed by this metapop. And there is, of course, the sentimental and nostalgic element. PolyGram just up and rereleased all the ABBA albums. Go treat yourself and buy one per month or something.