I find the search works more easily if the individual threads are short. It’s also a lot easier to find what you’re after when the message subject accurately reflects its contents, too, but this system doesn’t support mid-thread subject changes. Encouraging folk to start a new thread when they take the subject off in a new direction is likely to help, and having a fairly small limit on thread length is one tool to encourage that.
I wasn’t being entirely flippant (despite all the smilies) when I suggested a limit of…