I'm mostly in agreement here with Sam and oldkid. I read an article once that argued that roughly 45 minutes was the "perfect" length for an album, and listed Dark Side of the Moon as its exemplar. I tend to kind of agree with that, not on the bottom end, because short albums can have a great sense of urgency, but as a maximum, as there are so many albums that burn me out when they go longer than 50 minutes. Some are exceptions to that rule, and end up maintaining what you guys called "flow" all the way through. But that is somewhat rare. Even really good bands irritate me when they refuse to cut 2 or 3 songs and insist on having a 60 minute album that doesn't need to be that long (I'm looking at you, Modest Mouse).
The point on LP length and space restrictions is an interesting one too. Last year I named a double album my #1 record of the year (Baroness' "Yellow & Green"). It, like The Flaming Lips' Embryonic, feels sonically appropriate as a double album, but neither exceeds the 80 minute length of a single compact disc, so I can only assume the choice to split those was artistic, rather than practical.
I've become really jaded with the recent trend towards "Deluxe" edition albums, where the standard 10-12 song album proper is supplemented with an extra 2 or 5 or 9 "bonus" songs, most of which are usually throwaway b-sides. But, being a neurotic completist, I feel the need to listen to the Deluxe Edition just in case there are one or two great songs that didn't make the album. Sometimes there are, but it almost never enhances the album experience. It feels like a good trailer attached to a completely different movie.