One of the differences between 1973 and 1978 in rock music is that in 1973, rock music was wildly an art form that was basically still a rebellion for the most part; but sometime in the mid-70s, the paradigm shifted from being "art-based" to more "Corporate/arena based". For instance, your Black Sabbath, Pink Floyd, Led Zeppelin, Alice Cooper, Yes, early Genesis, even country-rockers like the Eagles (though they'd transition to the Arena style). The only real art form of rock music in the late '70s of course was Punk/Early New Wave, as the other non-disco rock was definately designed to be "made for radio" and then to be sold at tour. Kiss, Queen, Styx, Boston, Bruce Springsteen to name just a few. Touring became much more important as well. But what are some definate albums that mark this transition? (Kiss' debut is a great example).

Both however are typically under the same "classic rock" label however, when in reality we have two different styles.

Definitely. All I can say is that for each band you mention, there was generally one album that demarcated the artists' impending downslide. In some cases, it wasn't necessarily a bad album, but kind of preview of things to come. For instance:

Black Sabbath - Sabbath, Bloody Sabbath
More keyboards and prog style noodlings. From what I understand, the guys were under a lot of pressure from management and record execs for top-40 'hits', which is a really dumb thing to ask of Sabbath.

Pink Floyd - The Wall
The Floyd album most beloved by folks forty and younger is a tired and generally bloated retread of earlier Floyd tropes.

Led Zeppelin - Presence
Killer opening track to each side leading to...nothing much.

Alice Cooper - Welcome To My Nightmare
There was a lot more to Alice Cooper than the weird guy singing lead - namely the Alice Cooper Band. This 1/2-way decent album by an almost entirely different outfit began the long slow decline.

Yes - Relayer
It wasn't really until the '80s when Yes almost irreversibly traded in their cool Prog card for the MOR pop-rock stylings of Trevor Rabin, but Relayer clearly pointed in the direction of the watered down mediocrity that awaited years ahead.

Genesis - A Trick Of The Tail
Again, it was the '80s that saw blatant c o m m e r c i a l i s m rear it's smarmy head in the Genesis camp, but this 1st 'post-Gabriel' album is the real turning point.

Queen - Jazz
A steaming pile of 'hits'.

KISS - Dynasty
Have to disagree that their first album is the breaking point. Sure: they were blatantly commercial from the word 'go', and roundly lambasted for their apparent lack of musical finesse, but they penned some very powerful rock in their earliest incarnation nonetheless. However, by Dynasty, whatever meager ambition they had to truly rock had withered completely.