I would contend, though, that reasons we collect – nostalgia, the art itself, investment – will vary from person to person. I say that because I find myself frequently coveting and sometimes buying art from books I have no nostalgic feelings for. The most nostalgic era of comics for me is the mid 60s to the early 70s, but I have very little art from that period. I may be extremely nostalgic for JLA issues from the 60s, but make little effort to pursue the art since I don't care much for Sekowsky. That doesn't mean I wouldn't be happy to have one, but I wouldn't pursue it like I would something like a Ploog Kull splash which I covet for the art itself. So all those factors exist, but motivate different collectors to different degrees.
I totally agree, but just on the opposite end of the spectrum.
Using nostalgia as an example (as it is the basis of most of my personal collecting). I seek that warm fuzzy feeling of owning the original art to a book I remember reading as a kid one summer day when my grandpa would take me to the store to buy me a few comic books.
I don't care who the artist was that drew these images, (I really do enjoy Janson from his early Defenders work over Giffen though), I just rememember the images were powerful and interesting enough to draw my young mind away from all the others on the spinner rack.
Now while I love and appreciate Miller DD art (but don't personally collect the art to it), I do remember reading the original DD run off the stand, and at the time, didn't really notice any drastic changes in the art itself (perhaps because I wasn't looking for it), I just remember enjoying reading each new issue as they came out (I started reading off-the stands with issue #173 and read steadily until #191).
To me, this is where only nostalgia matters.
If I was to collect the art from this run, the only thing that would matter to me would be which individual covers/pages were burned into my memory the most from story moment images I enjoyed, rather than "artistic quality of the piece as a piece of art".
This is why I have so many Buckler, Milgrom, and Chan covers on my wantlist...sure they may not be the "greatest" artists of their time, but nostalgia is a powerful thing...