Backtagging:  I'm a slow tagger sometimes too! This is totally okay with me.
Threadjacking: I generally don't mind it. Ask if you're not sure though! 
Fourthwall: As long as it's not about Elementary specifically, I'll be okay with. Sir Conan Doyle's work is rather influential so it would be kind of difficult to skirt around it.
Off limit topics: I'm mostly okay with anything, but if you're not sure, it's always okay to ask! 

Hugging: If Joan knows someone well enough, hugging is totally okay. She's not the type to shove you away if you do it though.
Kissing: Generally reserved for people she's currently in a relationship with. But you're welcome to try? I'm not sure she'll reciprocate.
Flirting: I encourage it! Again, I'm not sure how she'll respond.
Fighting: Yes, but Joan isn't the type.
Injuries: General scraps and bruises, and occasional cuts are okay with me. She's a tough lady.
Killing: Let's talk and plot about this one! 
Mindpowers/Telepathy: If you contact me ahead of time, I'm okay with this! 
Shipping/Romance: I'm not against it, but I do love a good build up to it, and I'm a go with the flow kind of gal. If it happens, it happens. If not, Joan will be okay with that too!