Requested feature
Larger thumbnails.
Why would it be useful?
Larger thumbnails make it that much easier to determine the contents of the post without having to visit the page, making browsing and tagging easier.
Currently they're clamped to 150x150, but 200x200 would be nice, or perhaps 250x250.
Additionally, to fit more thumbnails on index pages you could reduce the margin between thumbnails as there is quite a bit of empty space between them.
Also, perhaps normalize all thumbs to a height of 200/250 px and allow them to be as wide as necessary, similar to this layout (with a cap on width for insanely wide posts).
This was suggested before (at least the size increase) but I'm making it official by putting it here.
What part(s) of the site page(s) are affected?
Any page with thumbnails.
Updated by Jugofthat