My opinion it depends a lot on the topic.

If it's something about job balance, content difficulty, or really anything else where the topic itself could be different from what it was in the original thread, start a new one. Likewise if the thread that exists is more than 6 months to a year old, also start a new one. People's opinons may have changed, and it could be unfair to people on the starting page to start getting replied to based on statements they made a while back that may or may not even accurately reflect their current opinons (or even worse could be from people who don't even play the game anymore)