I'll try to answer specifically. For me it is the body shape of the 99/900. It feels organic and part of the road, the mushroomed shape makes me feel like I'm part of a natural outgrowth of entire automotive ecosystem: map + city + road + car + driver + civil engineering. The interior is also minimal and of the school of Bauhaus or Dieter Rams. The late 900's and 9-3 lost this charm and became more conventional.
There's no logic behind it, it is simply shape and form that appeals to certain people. I know people who are nuts over early BMWs, or 1950's VWs, everyone has their thing.
Then he had the nerve to criticize for buying a Corolla, after teenage me got sick of dealing with auto shit after going through no less than three old, cheap Volvos that seemed to constantly need work of some kind. (Two of which had a habit of stalling at the most inopportune moments, like driving 60+ MPH on the freeway. It took years to feel safe driving again!)
To bad the company tanked. They had neat design going: http://www.retrotaksi.fi/en/index.html
I drove one, it was fine.. (Un-remarkable, but at the time I had a 1989 GTI, that was pretty fun...)
I remember having trouble removing the key from one (Its next to the shifter... And it needed to be in reverse or something.).
They are quirky and aesthetically distinguishing, especially all the orange and green.
Love my '95 Saab 900 SE turbo and will most likely never part with it if I can avoid it.