This morning I tried doing a desktop share with one of my coworkers and received an error I hadn't seen before:
Sharing is not supported with this contact.
It was odd because the user was Available and not using a mobile client as far as I could tell from the presence. Turns out, the user was signed in to OWA and using the Lync integration there to chat with me. It would be nice if in a future update (hint, hint) that an OWA user's presence be published as "IM Only" to indicate it does not support the other modalities.