Generally, when the last post in a thread is several years old, it is better to start a new thread, than to try to resurrect the old one. Times change, people move on, new items come on markets. Answers to questions may even change according to the season of the year.
As far as what constitutes a good thread, or post, it usually should be informative, may contain pictures, and be readable. The saying goes that a picture is worth a thousand words. This is not to say a post or thread has to be all business, either. Many of the more popular threads and forums members go off on tangents, and head to places never dreamed of when the thread first starts. (not that I'd know anything about going on tangents)
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.