5.x,6.X,7.X includes BLF
6.X DB read/write in Subs for user-front facing features
Third party and Cisco SIP Phones support
7.X G.729 SIP trunk, Exchange 2007, T.38 support
6.X Includes Mobile Connect and Mobile Voice Access
Linux based :D (No more Windows Update or Microsoft Office install in CCM :))
Your Cisco SE should have bunch of presentations