Since friends and my self have ventured into full body tracking recently I've noticed issues with it's implementation.
--- The thigh should not be rotating 1:1 with the ankle. This has never been how the human body works. Most characters are not weight painted for such abusive twisting in the hip.
--- The head vantage point of the avatar sinks into the neck or chest when sitting or laying down. This is pretty important and problematic since one of the entire gimmicks of full body is to be able to do this in VRchat.
--- Calibrating every avatar when you change to is silly specially if it's an avatar you used in the same session. Allow for profiles or store the information with the avatar.
--- Proportion adjustment is necessary. It's obvious at this point forcing realistic proportions onto fantasy characters with limbs that are various size is a problem. Even for standard VR users. Arm length adjustment without effecting your player height is very important. This can potentially solve the head sinking into the chest when sitting or laying down.
--- Avatar arm IK freaks out when laying on your back and reaching for the sky.
--- Clavicle bones are not being included in arm IK. It is for standard VR users. Why is full body IK not including this bone? It makes avatars look silly.
--- Location of player height button and scaling the world as you change it with a "EXIT VRCHAT" button next to was probably not the best design choice. Perhaps informing desktop users this does nothing for them would be helpful too. >_O