This site has a necro-thread problem. Old questions get commented on (often in a questionalble manner).
Perhaps SE in general needs a way to graphically show the age of the thread to give you a warning? I often mistakenly comment on or answer obviously dead threads before I think to check the question date.
EDIT: To clarify - How can we prevent OLD threads from being mistaken for NEW threads.
Old threads often have an acceptable answer, but are not marked as answered. Or, even if un-answered, the discussion has died out. Therefore, any comment or answer is probably wasted effort, because the original poster has long since lost interest.
This is the real problem. When an old thread is mistaken for new, one answers it in all seriousness - perhaps spending a non-trivial amount of effort, but the asker never sees the answer.