Hi, Percy. In the grand tradition of brainstorming, I'll inflict some random thoughts.
I like having an activity meter, but wonder if you need to register activity with both the progress bar and the color change: maybe some other already present element could change color with activity.
I have noticed that the bar sometimes fills completely when a topic is busy, which looks odd; and at the low end the correlation between the progress bar and number of messages/message rate seems unclear. How do you calculate activity for the bar?
How about the third column from the left, showing the topic mood? I frankly don't get a lot of useful info from that column, though I like clicking to a msg index.
Maybe the current symbols in that column could use color changes to indicate activity, without using a bar. If you did later make a vertical bar, that might be a good spot for it, and it could still click to a msg index.
Alternatively, you could make that column display an actual meter (perhaps as simple as just the meter's needle pivoting left to right while changing color), perhaps still clicking through to a msg index, with the column head clicking to a sort by activity level, though I suppose the All Topics list does that pretty much since the most active threads will have recent msgs.
I do like Granny's elaboration of your idea about different colors, though I'd probably make dark blue the low level of activity and bright cyan the high.
NB: I was reminiscing with someone about how difficult it used to be to work with a team when coding database and/or document management software, keeping everyone up to date through countless iterations...I thought of your section, thread and intra-thread msg/reply structure, and it occurred to me that what works so well for a forum could also work well for a project, whether the team be coders, technical writers, or a bunch of perverts writing a new Naked Lunch any sort of text- based collaborative effort could benefit from your structure.
Have you ever been to an American wedding? Where's the vodka? Where's the marinated herring?! -Gogol Bordello