Just a few things to keep in mind:
1) 90% of policy, you can learn from reading the Terms of Service threads.
If that's too tough to read, then it really boils down to this: be nice to people (no flaming). And if you don't like the show, don't watch it. (trolling)
2) What's off-topic? Given this is a Grimm discussion forum, TV.com specifically created blogs so folks could have games, personal conversations, fan clubs, chats, etc. Any and all such games, chain-type threads, lounges, fan clubs, etc. can and should go to blogs.
Discussion of TV.com and forum policy is off-topic on the Grimm discussion forum since they don't concern the Grimm TV series, characters, storylines, etc. If you want to discuss policy, you can PM the show editor, and after that the general policy forums.
The occasional straying post is not a concern, but extended personal discussions and thread hijacking are.
If you wish to advertise personal sites, fan clubs, petitions, etc., include them in your signature or your blog, not on the forum in your posts and threads.
3) Spoilers: Make sure to put Spoilers in your topic header if you have spoilers. If you must put a spoiler on a post within someone else's thread, please use spoiler tags. If there is no thread for a future episode, direct discussion to the Season # Spoilers/Speculation Thread.
4) Please don't create Official Discussion Threads. I'd rather avoid either races or competing threads. (Yes, some people like to race to do it: I don't know why.) This also means all general and specific discussion of the episode goes in the episode thread. This can be inconvenient, but so is a forum filled with dozens of individual threads about a single episode.
5 ) When creating new threads, please try to make them discussion rather then polls, so we don't end up with tons of one-liners. thus "Who is your favorite character?" - not so good. "Who is your favorite character, and why?" - better.
6 ) No need to post a "Hi, I'm new" thread. Just jump right in and start posting.
7 ) Reviews go in the review section, for the episode or the show.