Thats right about software support, Siemens will not write patch for old software, mind you, if you have not got a problem then that's OK then. Going with the Microsoft analogy a version 6 isdx would be like NT3.51 or Win95
As for features, some that come to mind are -
Hotdesking, home working, networked ACD, Networked keysystem
Voip – Desktop , trunking , SIP carrier support soft clients
user selectable CLI
Auto BST/GMT time change , network time support (sntp)
call deflect to voice mail
There are some handy new commands including lucf/rucf list and revise user controlled features (diverts DND etc) lecs leta list extensions by cos or tac.