P2P streaming music program Spotify is expected to launch in the US later this year - it is currently open to users in Sweden, Norway, Finland, the UK, France and Spain - but Billboard has been testing the service for the last few weeks. The service is a fantastic way to listen to music and it differentiates itself from its competitors with a stripped-down design and speedy reaction. It has quickly amassed about three million users and has been widely praised by users, analysts and journalists. Because Spotify is winning over so many fans, the new service acts as a case study for the role discovery plays - or doesn't play - in the success of online music.

Put simply, simple works. Spotify just plays and organizes music and the user interface is basic, easy to use and free of even the slightest load time. The layout is far less cluttered than those of other services. Not much emphasis is given to keeping users fingers on the pulse of the music world. Radio station options are simple as well. There seems to be great value in serving music as simply as possible and Spotify follows a long line of other applications that have been heavily praised for doing that: iTunes, P2P apps, and many mobile music apps such as Pandora, Imeem and Last.fm.

"It doesn't have the discovery, search and community functionality that we've come to expect from the streaming services," Forrester music analyst Mark Mulligan previously told Billboard's Antony Bruno. "But that's why it just works."

And judging from the list of most popular albums at Spotify, its users are content to be less venturesome. On Tuesday (June 23) Spotify's top 100 albums list was heavy with older albums - more old albums than are found on its competitors' top lists. Best of collections are scattered through the list: "The Essential Bob Dylan" (No. 14); Queen's "Greatest Hit" (No. 18) and "Greatest Hits II" (No. 57); Bob Marley & The Wailers's "Legend" (No. 80); "The Best of David Bowie 1969-1974" (No. 81); "Absolute '90s Vol. 2" (No. 86); "The Definite '70s" (No. 87); "Cross Road: The Best of Bon Jovi" (No. 95). Many studio albums make an appearance as well: The Offspring's "Americana" (No. 33); Nirvana's self-titled album (No. 35); Green Day's "American Idiot" (No. 44) and "Dookie" (No. 49); The Killers "Hot Fuss" (No. 53); Michael Jackson's "Thriller" (No. 78).

The tracks list is slightly less oldies-driven but shows Spotify users are not listening to hits at the same frequency as users of competing services. Coldplay's 2003 song "Clocks" is at No. 33; Green Day's 1995 hit "Basket Case" is at No. 44; Queen's "Don't Stop Me Now" is at No. 60; Green Day's "Good Riddance (Time of Your Life)" at No. 68; Nirvana's "Smells Like Teen Spirit" (No. 71); Bryan Adams' "Summer of '69" (No. 88); and the first two singles from Hot Fuss make appearances, "Mr. Brightside" (No. 94) and "Somebody Told Me" (No. 100).

Besides the simple design, some other aspects almost certainly play into the early success of Spotify. Its P2P-based technology caches data so songs start playing with an imperceptible lag time, just as with MP3 players. The speed with which album artwork appears, and the way a song or album is dragged without lag or friction to the application's sidebar, are small differences that make Spotify feel better than its peers.

If Spotify is popular in part because it doesn't push music discovery, companies should re-think the role discovery plays in services that are meant to reach mainstream listeners. Perhaps early adopters, those most interested in finding new songs and being exposed to new artists, desire music discovery more than later adopters. Even the technologically adept, however, seem smitten by Spotify's simplicity. Maybe the first successful music streaming service will be less about discovery and more about listening.