That said, both Watchmen and Knightfall were published as single issues before going to the tpb format. The Killing Joke, on the other hand (to find the point of intersection between both these stories haha), was always a Graphic Novel: a single, longer story printed in a premium format. The Killing Joke is also something like 64 pages long.
Many great things already said (@Rodimus13 great short version paper! haha) but I'd like to throw my 2 cents and reiterate that the difference to me is:
1) One of format, having less to do with a clear beginning, middle and end structure and more to do with being published a certain way. A Graphic Novel is a longer book, usually printed in better conditions, published as such. The format does make the start/middle/end structure easier, though, if not even somewhat logical.
I wouldn't consider most tpb collections as Graphic Novels, but they sure can be repackaged and remembered as such. A good example of this is the recent ''Bad Weekend'', by Ed Brubaker and Sean Phillips, to be released in July. It's a new packaging of a two-issue story arc published in their monthly comic ''Criminal'', which was so well received they are republishing it as a GN, mixing both issues and adding extra content in one package.
2) A publicity stunt. As with any art form, comics had (and still have, in a way) to struggle to assert itself as art of value. The easier way to do something like this is to ape on an already established art form, therefore begins the ''Graphic Novel''. It's just like literature! Look how mature we can be!! This evolved a little, with the GN being more a format now than before, but the ugly stigma of the word ''comics'' still applies today to some people, so from a marketing standpoint it sure makes more sense to use Graphic Novel as a term.
Graphic Novels are just comics, to me.