I think that making Hot Threads the default sorting option is problematic because it basically hides new or updated threads from anyone who is not paying attention to their sort settings. Personally I only noticed this setting when I posted a thread and found myself wondering why it was not visible in the forum topic. Hot Threads is good as a manual sort option but the fundamental problem with using this as default is that no new thread can ever become a hot thread unless it is visible enough to get replies in the first place and since we all know very few users ever go beyond the second or third page on a forum this has the potential to prevent a lot of threads from getting views let alone replies