Went a different way with shortcodes. Now they all run at priority 11. Shortcodes that have a buffer line between them and surrounding code aren’t given a paragraph tag wrapper, so you can roll your own formatting for block-level shortcodes. Anyone who has developed a shortcode plugin for 2.5 should see what their output looks like on /branches/2.5/ now.

With the help of AaronCampbell, the shortcode HTML issues have been fixed in a flexible way. Now add_shortcode() takes a third (optional) parameter that allows you to have your shortcode be expanded after wpautop() and texturize() are run. Default is before for compatibility with 2.5. For “block-level” shortcodes, you should probably make them run after. For “inline” shortcodes, you should probably make them run before.

Update: We went a different way with this, for the time being.  Now everything runs at 11, but shortcodes with a buffer line separating them from other content will be recognized as block level and won’t get paragraph-wrapped.