What the difference a default makes. 

Upon joining Microsoft I know have the pleasure of using Office Communicator as my primary IM client, as opposed to Lotus SameTime while at IBM.  Nothing inherently wrong with that since basic IM functionality is pretty well established across the board.

The interesting thing is how Office Communicator handles presence information - in particular integration with one's Outlook calendar.  By default Communicator looks into your calendar and, if you are scheduled to be in a meeting or otherwise occupied, sets your presence status accordingly.

In theory this is a great idea and offers tight integration between calendar and IM functions.  In practice it has the effect of making one seem more busy than perhaps you really are.  Meetings don't always happen as planned, or take as long as planned - similarly, assuming I have my laptop with me I may very well be able to exchange brief IM messages, as opposed to phone calls or e-mails.

Maybe it's just me, but I find this default behavious acts as somewhat of a collaboration disincentive.  Of course I've now learned to just go ahead and "ping" people anyway - even if they're shown as busy.