all of the above. A cohesive album has a good sequence- the best have the "right" sequence (I know that is subject to debate, even for iconic albums like Pet Sounds), and the tracks enhance each other.

One classic formula from the vinyl days you see pretty close variations of: a propulsive opening, expression of the album theme, a reflective piece, a piece that ends uncertainly; then, side two, a more hopeful or reflective song, followed by more complex tunes- both quiet and loud- that build to a catharsis which is resolved at the end of the side. You see it everywhere, in classic rock: Abbey Road, Synchronicity, The Unforgettable Fire, Sticky Fingers, 2112. Maybe not exact blueprints but they stick to a general format.

I was going to say some kind of general sound cohesion helps, too, but it's not essential- check out Neil Young's Tonight's the Night- most of it dashed off in some spontaneous sessions at one studio, but since the theme of the album is remembering fallen brothers, sticking a significant track from an older live recording (Come on baby let's go downtown) is poignant and perfect for that moment.

Variation is pretty essential, imho- but if the music is powerful, there are certainly exceptions. One could say Wish you were here by Pink floyd is dominated by one really long blues workout broken up over a couple of tracks. But it never (at least to me) feels boring, and it makes the title track and "Welcome to the Machine" stand out in stark relief. Jimi Hendrix uses the same trick on Electric Ladyland-sprinkled amidst one long blues number ("Voodoo Chile") and a dreamy side 3 ("1983" and "Moon turn the Tides") are laser-sharp rock songs that seem all the more exciting in contrast.

So there really is no magic formula, except perhaps that you took the time to plan it out