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Further investigation shows that none of the most popular forum suites do this!? I checked phpBB (
demo), punBB (
demo), Invision PowerBoard - threaded is the same as newbb (
demo, Phorum (
demo)...
Some that look promising: vBulletin - has 'hybrid mode' that looks good (
demo), drupal - has nested view that looks nice (
demo).
What I want seems to be standard for comments but is obviously rare for forum posts. slashdot has the best version of this using their score system (
demo) and my previous favorite portal software geeklog also does a good job with comments (
demo).
I will continue my searching...