Also scripts should not require conditionals so you'll have to write to the portable subset. Various issues are analysed. Even how a line is drawn is different. Mac has a pen concept which is different from X's. We must provide a higher level interface to these dialogs and Tcl will call the one for the os it's running on.
Menus are different.
Tk will have to decide where to place them based on the system. If you change the object, it gets updated wherever it appears. Font requests should never fail, something else should be substituted. Various questions about slashes for directory separation, size of integers etc. If you find any, let us know.