I don't know about America, but from what I've heard progressive rock was huge in 70's. The Dark Side of the Moon, which was released in 1974, being the 2nd best selling album of all time proves it, I think. Actually, I'll tell you a story to back it up. Here in Turkey back in 70's you wouldn't easily find any foreign records. You'd hear a song on the radio, you wouldn't know its name most of the time. My mom, who was born in 1967, remembers hearing King Crimson on the radio in early 70's. Funny thing is, she didn't know the band. She has always been a big fan of the song "Epitaph", but she thought it was sung by a man named King Crimson for years. Then in last year when I was trying her to get to know the stuff I listen to, I opened the song The Court of the Crimson King, which was on the same album as Epitaph, In the Court of the Crimson King. My mom remembered the voice, asked me if there was a song named Epitaph by the band and there we go, she learned the name of the artist she was listening to after 35 years. That's another evidence that progressive rock was huge in 70's, even in a country like Turkey which didn't even have TV's until 80's, people knew progressive rock songs from that era.
Pop has always been there, because basic, close-minded, stupid people has always been there. (I know I'm going a bit harsh, but hey) Disco was huge in 70's, electronic/synthpop was huge in 80's. Hair metal had its strike in late 80's, grunge crushed hair metal in 90's. Hiphop had its foundations in 80's and started to reach its fullest limit in 90's. (in quality)
Then came RnB in 2000's. Rap lost its underground edge and became mainstream which led to horrible, sex/drug/sex/drug/shake that ass material songs to dominate the music world along with boring comtemporary RnB songs. Comtemporary RnB has had power since 80's, but it's nirvana has been in the 2000's. (not by quality, of course)
And now in 2010's, electronic music, dubstep has been the biggest genre of music. RnB still has its power with the same mainstream artists. I think the recent rejuvenation of electronic music is related to that of 80's electronic music and the patterns used in the new mainstream music, such as beats in hiphop songs. I expected such an attack by electronic music.
I'll try to list eras/popular genres by complexity/straight-forwardness.
1970's Popular Music
High level : Progressive Rock, Early Hard Rock
Mid-level : The Beatles
Basic-level : Disco
1980's Popular Music
High level : Hard Rock
Mid-level : Glam Rock
Basic-level : Electronica / Synthpop
1990's Popular Music
High level : Heavy Metal (barely, metal had a breakthrough and went mainstream with Metallica's success, especially with the single One, but the general quality of the genre lowered in the same period, coincidence ?)
Mid-level : Grunge
Basic-level : Pop bands (girlband-boyband)
2000's Popular Music
High level : Nothing, actually.
Mid-level : Alternative Rock
Basic-level : Comtemporary RnB & Cheesy Hiphop
We're in early stages of 2010's, so it wouldn't be fair to write about that in general. Though the basic-level carried on with comtemporary RnB and cheesy hiphop, with newly added dubstep and the turnaround of electronica.
Electronica probably will carry the torch for 2010's. I'm very curious about 2020's popular music, because a sharp turnaround can be expected. Though as the society continues to get dumber, maybe we'll see even more horrific stuff on mainstream.