Just to get this out of the way. Most things are pretty self-explanatory~ :3
1. You have to be a fan of the L/Misa pairing. xD Blatant and obvious. It's totally cool if you ship other pairings like L/Light, or Light/Misa, but at least like/support L/Misa. <3
2. Don't trash other people's stuff. Okay, so maybe you can do a better drawing or write better fic, but that doesn't give you the right to make nasty comments about something that someone worked hard on. We're all L/Misa fans here and it's really hard to find anything fan-related for the pairing, so we've got to pull together and appreciate what we've got! And then make more stuff. xD
But seriously, if people want criticism, be constructive. Tell them what you like about their writing, what they did well, and then say how they can improve. ^^ It's really helpful that way.
3. LJ Cuts are love. <3 You know it. ;D Put stuff (fics, art, etc) behind a cut (or fake-cut out to it, whatever) just so it doesn't clog people's windows who don't have fast internet.
4. Keep it DeathNote-related. I don't think this is going to be a problem, seeing as this is an L/Misa comm~
5. Have fun! Not really a rule, but still~ we're here to have fun, so let's do it. 8D
As a side note: anything fan-based is allowed-- so fanart, fanfic, icons, doujinshii scans (so long as you give credit to the artist) are fine.