Had to think about this for a bit. From a software perspective, there are only 3 programs I typically use that are not SONAR (in all stages of work). Only Ozone for mastering specifically, and the "best feature" for me is that I can save one preset that includes the "whole ball of wax." This thread also made me realize I have not used Mix Recall yet (doh!), so not sure all that it does (so this feature may already be present now). As far as components within Ozone, Craig nailed it that most can already be done within SONAR now.
The noise reduction (and other utility) functions I often run after tracking as destructive edits, but as I was thinking on this I realized that RX4 is also iZotope. I use Audition just because I have it, but RX4 is probably a far superior product.
Then it occurred to me that both Cakewalk and iZotope are in Massachusetts ... what a great "partnership" that would be

(All the "missing mastering goodies" can be found in one place, why recreate the wheel?)