I can't find where to view cinematics that I have "unlocked" (by having them occur naturally for game progress). Am I just missing it?
If you go all the way back to the login screen, there's a Cinematics button. You don't even need to log in.
If you can't see some cutscenes(possibly due to a corrupted/missing cinematics file), this thread details a workaround that unlocks all of them:
Open My Documents\Diablo III\D3Prefs.txt (for win7/vista, not sure where it is in xp or os-x) in notepad and change the "PlayedCutscene" lines to (or add them if they are missing)
Also right-click the file and check the properties so it isn't set to read only for some reason.
Note that this unlocks all cinematics so if you don't want to spoil anything don't watch them all :)
Make sure the game is closed before you do this.