Quoth the Raven, "BRAIN TUMOR!"
This article posits an interesting hypothesis regarding the death of Edgar Allan Poe, and brings back memories of my American Lit survey courses from sophomore year.
I remember 2 things clearly from that course, taught by a specialist in Poe:
- Edgar Allan Poe didn't go by Edgar Allan Poe, he was just Edgar Poe. The Allan got added in posthumously.
- My professor was convinced, based upon this daguerreotype , that Poe died of a stroke. He always noted the way that one side of Poe's face drooped.
If it turns out that Poe did suffer from a brain tumor, then perhaps my somewhat eccentric, banzai-tree-pruning Poe specialist professor (who is still at Davidson and still teaching, I'm pleased to note!) was at least partially correct - brain tumors can be a factor in making strokes more likely to occur, after all.

You can't make a lot out of somebody's expression in a daguerreotype: the long exposure pretty much rules out anything except a sort of grimly frozen look. Try holding a smile for thirty seconds and you'll see what I mean.
I suspect our modern view of the Victorians as somber owes a lot to the limitations of early photography; certainly the drawn illustrations of the day (for instance, those originally published with Dickens' works) show a lively bunch of people.
PfP - I think his point was that Poe had to keep a "neutral" expression for the length of the exposure, and that his neutral expression showed evidence of droop on one side, which I can sort of see.