Letsrun.com: Composed primarily of above average runners who have become jaded as they realize that they do not have the talent to compete with the best. The forum is tolerable only because it contains the most specific posts regarding collegiate and professional track and road events. If you can get past the obnoxiously rude posters who lob sarcasm from behind a shield of anonymity, kernels of knowledge can be found. Just be prepared to come with a hard hat and pick axe to get through the crap.
Runningtimes.com/forum: A step below letsrun.com in terms of athletic ability of the posters and the content of most posts. Runners here tend to be age-classers who salivate at the chance to cherry-pick a local 5K to garner that 2nd place 45-49 10$ gift certificate to their local skips dealer. A handful (and no more) of posters are quite knowledgeable and give thoughtful and thorough responses. However, these same posters are crotchity sots who hate the fact that they are relegated to posting in the second level of hell.
Forums.runnersworld.com: A complete waste of a site. If you can't read and like pictures, this is the site for you. A schnazzy avatar and an air of wisdom will win you a legion of minions who just want to check that "I want to run a marathon before I die" goal off their life-list. These beginners and runner wannabes don't have a clue what they speak about, but that certainly doesn't stop them from wasting server space with inane posts. Avoid this forum at all costs. There is nothing to be learned there and no redeeming entertainment value.
Coolrunning.com: From what I have observed, I believe this is where society's micreantes and cretins go to discuss everything but running. Based on the posts I have read, I assume Jeff Galloway and the walking penguins have successfully overrun (so to speak) this place. A step below runnersworld. A big step below.
Runango.com: A relatively new player in the online world, and I hope it is short-lived. Composed primarily of runnersworld outcasts, this motley crew jumps at the chance to break out into a group rendition of kumbaya. The cliques are so tight that they make Joan Rivers skin look like a woman in her 70s. Another place to be avoided.
Where does that leave one? Unfortunately, here.