For me, Spotify is a sub par experience because I don't know what I want and I have different music tastes than my friends. The only thing that makes Spotify useful for me is sites like http://sharemyplaylists.com/. I imagine for people who know exactly what they want and just treat it like a giant jukebox, it is a fantastic service. I am not one of those people.
Napster for me, was about finding people as much as music. I was part of a community of people that were super stoked to talk about some new indie band. I would search, notice who had music I was looking for, send them a message and start talking.
I met a lot of people in real life through Napster often at shows. I know others did as well since we received thank you letters and in a couple cases, wedding announcements.
I think the community with its passion for music was what made Napster great, not the massive catalog. All I had to do was enter one of many indie channels and read for half a minute before I had three new things to listen to and a bunch of people to talk to them about.
Not to discount the catalog, I will say that the catalog at Napster will possibly never be duplicated. There were a lot of back catalog works, a lot of pre-release works and a ton of bootlegs. Sure, the quality was sometimes poor, but then I used Napster as a tasting service and then, you know, bought the originals when I could.
My hope is Spotify continues to improve and eventually becomes more community oriented or something else comes along and re-ignites the flame. Music is one of those things that is central to a lot of people's lives and you couldn't ask for a more passionate userbase.