Share 'Asterisk vs proprietary IP PBX systems'

I'm looking to start a discussion and hopefully get a long thread of comments specifically regarding technology specific benefits Asterisk has over various proprietary systems. I don't want cost benefits, I want definitive comparisons between the technologies themselves. For example, how does an Asterisk IP PBX compare with a large Nortel, Avaya, or Panasonic system? Don't think Switchvox or T...