For those who don't know, one of the major changes in Tk 8.6 (currently in beta), is that the Mac version has been moved from Apple's older-and-now-deprecated transitional Carbon foundation, to the more native Cocoa libraries used by the vast majority of Mac developers. If you (or your users) work on Macs, this should be a significant improvement.
This work has been done by Daniel Steffen, who deserves our thanks for this. If you're interested, Kevin Walzer has recently posted a series of blog entries about Tk-Cocoa talking about his experiences. I'm sure Daniel and the rest of the Tcl/Tk core team could benefit from additional testing and feedback on this new work, so if you have the opportunity, please try it out on your own code.