Jonny Come Lately
The New Kid In Town
For me, the seventies belonged to Kansas, Yes, Styx, Springsteen, Rush, ABBA, Fleetwood Mac and Boston.
ABBA? Each to their own, but I am surprised to read a post on a rock forum citing them as one of the greatest acts of the decade. To me, they have more in common with bubblegum pop stuff like the Osmonds than they do with any of the other bands you mentioned - their songs have nice (if rather sugary) melodies and are sung nicely but most of the time the music might as well not be there as far as I'm concerned, and it's not like they had the lyrics to compensate (to choose an especially well known example, when was the last time you saw a tambourine in a disco?).
Comparing ABBA with Rush is pointless as they are so clearly different, but if you compare them to late 1970s Fleetwood Mac for instance, Lindsey Buckingham is a great guitarist and blows the ABBA guys out of the water on the musical front, Stevie Nicks sings with a passion the ABBA singers can't match while Mac while John McVie and Mick Fleetwood make a strong rhythm section. The gulf between them is perhaps best illustrated by their live performances - I have seen and heard Mac delivering great band performances whereas whenever I have seen ABBA on TV there is nothing to it other than the vocals.
To use another comparison, although I am not a great fan of The Beach Boys I can clearly see the artistic merit in Brian Wilson's compositions and their songs were very well composed and arranged, whereas ABBA's music is just 'there' to give whoever is singing something to sing along with.
Just my opinion, of course.
I disagree with Led Zeppelin being overrated (I think they deserve all the credit they get) but can sympathise with your views as I am not a Beatles fan and therefore know how it feels to not love such a popular and acclaimed band.