They say that disco died on July 12, 1979, the day radio DJs held an anti-disco demonstration in Chicago during a baseball game where they collected disco records from the crowd and blew them up in the middle of the field. It was a dramatic publicity stunt that signaled the public's shifting taste in popular music. But "died" is a rather harsh and really inaccurate term. Sure, disco was in decline, but it wasn't dead--not by a long shot. The second half of 1979 and 1980 saw disco tracks by major artists continue to top the chart, like Michael Jackson's "Don't Stop 'til You Get Enough," Donna Summer & Barbra Streisand's "No More Tears (Enough Is Enough)" and Lipps Inc's "Funkytown." Disco was still hitting #1 as late as mid-1981 with the "Stars on 45" Medley.
So that Diana Ross would release a disco pop album in 1980 wasn't really so unusual. Despite having limited success with disco during its late '70s heyday (She hit #1 in 1976 with "Love Hangover," but had no other major hits until this album), Ross landed a major hit with Diana, which became the biggest-selling album of her career. Its first single, "Upside Down," was a #1 hit in the US and #2 in the UK. Other singles included top 5 US hit, "I'm Coming Out," which was later appropriated as a gay anthem, and "My Old Piano," a top 5 hit in the UK.
Musically, the album owes much to its producers, Chic's Nile Rodgers and Bernard Edwards, although the final cut differed significantly from what they produced. Best known for their #1 hits "Le Freak" and "Good Times," the duo excelled at the funkier side of disco, and that sound is evident here, where silky string arrangements blend with funky guitar melodies to form an unmistakable disco sound. "Upside Down" and "I'm Coming Out" are perfect examples of this, with the guitar being particularly prominent, while the strings shine more on the second track, "Tenderness."
There was apparently a rather significant clash between Ross and her producers over this album. According to Wikipedia, he original cuts didn't turn out that great and its a he-said/she-said issue exactly why. From what I've read, they accused her of singing flat and she accused them of burying her voice with the album's funkier musical elements. Ross took the album to engineer Russ Terrana, who removed the songs' long instrumental sections, sped up certain elements and worked with Ross to re-record vocals. The Chic producers were not party to this and, although they objected, they didn't go as far to demand to be taken off the album's credits. The original Chic versions were later released with the 2003 remastered version (I have only the original version, so I can't comment authoritatively on how they differ, although I have listened to the previews at iTunes and there does seem to be a tonal problem between the vocals and the background).
Ross continued churning out hits in the early '80s, most notably the #1 duet with Lionel Richie, "Endless Love," which spent 9 weeks at #1 in 1981. Rodgers continued to produce very successful albums in the '80s, including David Bowie's Let's Dance and Madonna's Like a Virgin, while Edwards went on to work with artists like Duran Duran and Jody Watley. Despite the animosity with Ross, both producers would eventually work with the singer again.
Best: Upside Down, I'm Coming Out, Tenderness