My aim was to display all the relevant meta-information cleanly, and specify enough leading to improve readability, but no so much as to waste too much screen real estate. It works especially well in Widescreen View (not surprisingly, since that’s what I use).
Download Mailx here, put the unzipped style in ~/Library/Application Support/NetNewsWire/StyleSheets/, restart NNW, and select Mailx as your style.
Update: thanks to Isaac for pointing out that installation is easier than my instructions: unzip and double-click; NNW will do the rest.
Update 2: I’ve added a bar on the right of blockquotes to make them more obvious when there’s an image on the left that obscures the left bar.
Update 3: I tweaked the colors just a bit for compatibility with the rest of NNW’s appearance.
Update 4: I added a little left & right margin to images.
Try it; you might like it.
Here, for comparison, are Mailx and Mail. The font is Lucida Grande.