WordPress has made many changes since I joined and often my theme choices have been due to those changes. I began, I think, with Oulipo, just enjoying the way it scrolls and the simplicity of it. I left it only because the header is far too short.
A bit similar to timethief, I try to choose free themes, although I dearly love several "premiums" and am currently using "A Simpler Time", I think because it is darker, but not black/grey, and because it includes a touch of red. I probably am drawn to red a lot, but would not choose a simple theme to color the background red--too much.
I will never choose a white theme and leave it white. A purely "white with touches of neon-cobalt" site hurts my eyes after a while, for some reason, and seems coldly clinical, or something. Adding only a pale taupe paint job to the background of an all-white site truly helps all of that. "Clean Home" is gorgeous done that way and I am thinking of going there.
"Adventure Journal" appears on one of my other sites, and I love the casual look, there, (may I say 'humorous but not girly'?) and the total ability to change the header and background. Removing the paneling background and painting it to highlight my header photo is fun, for now. My readers are in to fun, and actually seem energized when they notice I've made a bit of a change.
I have a couple of unpublished sites I use for testing various themes that interest me.
I have probably talked a friend into using "Confit" for a "Website" for the camp she manages, replacing the food photo with a beautiful scenic photo of the campgrounds in Spring. Also, the map widget, she loves. Or she may go to Fiore because the camp is named "Cherry Hill" and the artwork seems to satisfy her. I'm worried about the widget area, there, though, for newcomers, which most of her hits might be...
I must add, I wish WordPress would offer more opportunity to view sites grouped by theme. I know sometimes we get to see maybe three themes compared, but I'd love just gazing at sites, picking by thumbnail (or not) according to their themes, to find inspiration. I suppose that would be a monumental task? Seems there could be even a search window to look up sites by theme?
Thanks for everything WordPress provides. It is amazing, friendly, helpful, and totally cost-effective. :) Why doesn't everyone use WordPress? I'll never know!