It's annoying when someone bumps a four-year-old thread with a reply to a specific poster and seem to assume that the poster held the same opinion they did four years ago and is still primed to debate the topic. Those posters are often not even around anymore.
Otherwise, I don't care. It jumpstarts some neat discussions when people have new ideas on old topics.
Sometimes, long threads can be annoying if there's been a conversation between a few posters for several pages and it's impossible to determine how the conversation evolved, but reviving old long threads can breathe new life into what the topic was really about.