I am really pleased that you have started this thread. I too am struggling with the designing of 'appropriate' activities in SL. Here are some thoughts/criteria:
(a) activities that exploit the potential of physical presence (in the virtual space); that is, being able to see and work with each other synchronously via avatars. So if the activities which are being performed remotely and involve decision-making or negotiation, it is best to meet in the virtual world, talk about the issues at a scheduled time, and make notes via the 'chat' or IM, and hopefully, reach a decision in this synchronous mode of communication.
(b) socialisation: this is the key for successful collaboration. If the students/participants are able to meet in the virtual world, share backgrounds, expected outcomes from the activity, how they would like to perform / schedule the collaboration, etc. - it might be a more effective means of socialisation at the start of a collaborative activity than aysnchronous e-mail interaction with a group of fellow-students.