America, rather than being a "poor man's Eagles", as a poster above suggested, were probably more like a poor man's Neil Young, at least in the beginning...except they were better singers. They weren't all countrified like the early Eagles, and they didn't have that rock edge that the later Eagles had. They had a decent string of hits that were pleasant to listen to.
The Doobie Brothers were sort of a good-time boogie bar band. Songs to play while you're on a road trip type of stuff. Great, catchy tunes. Their sound changed dramatically when they added Michael McDonald to the lineup and became the personification of what is now known as "yacht rock".
Dr. Hook and the Medicine Show began as pretty much a novelty band, singing a lot of songs by Shel Silverstein. Then they dropped the "and the Medicine Show" and just became Dr. Hook, specializing in fairly bland, soft pop-rock. I only like them for the nostalgia factor.
Flying Burrito Brothers were among the bands that pioneered the "country rock" genre. They were an offshoot of the Byrds, I think. They were critical darlings who never had a hit song. "Hot Burrito #1" is a pretty tune.
Loggins and Messina are my personal favorites of this list. They didn't have a ton of hits, only 3 or 4 really, but they had a ton of great album cuts, like "Watching the River Run", "Vahevala", "Peace of Mind", "Angry Eyes", "Holiday Hotel", etc. etc. They were musically probably the most versatile band on the list. Don't be put off by what Kenny Loggins eventually became in the 80's...this is when he was really good.
Bread is pure soft rock schmaltz...but they are very good at it. It just gets to be a bit saccharine after listening to more than two or three of their songs. Some of their songs are probably considered classic love songs, so good for them.
Seals and Croft usually managed to come up with a great tune every year or two. They were devout members of the Baha'i faith, so they were super serious and a bit pretentious, which probably kept them from becoming bigger stars.