It can be a problem for people with limited display space for the Subject: line. The significant part can be pushed right off the edge.
I suspect a lot of people don't recognize this issue because we've all become spoiled with big monitors and high resolution graphics cards. But there are still people reading their mail on PDA's and other small displays, and every pixel is precious. I also wonder what text-to-speech processing does with subject line decoration.
You might consider adding a filter for the existance of a List-Id: header whose contents haven't been recognized.
Cool idea. Gotta go implement that....
