Backtagging : Of course! Give me a shout at my plurk (saminabox) or PM this journal to let me know if I'm missing something!
Threadhopping: If it's not private with full encryption, then it's all game.
Fourthwalling: I'm okay with it, as long as it was discussed between players.
Offensive subjects (elaborate): We should talk about this first, I assume we would if it's offensive!

Romance/Relationship: Hank isn't a big flirt, but if you like this old man, he's well old fashioned. Dinner to movie dates and dessert and walking you home, all that jazz.
Physical Violence: Fighting and injuring? Not a problem. Killing? Should be discussed first
Mental/Magic Voodoo: Sure! It's not like that hasn't happened before.
General Warnings: None, but will append if it comes up often