Mozilla 1.3b: There aren’t many obvious differences – the theme I installed still works, which wasn’t true of 1.1 -> 1.2. One of the differences is the junk mail controls. They took about five or six days to ramp up, but now the only false positives I’m getting are cron messages. Another very small but useful item: you can now right-click on a link in Mozilla Mail and get the ‘Open link in new tab…’ option.
Gnome 2.2: Not many obvious differences here, either. I don't really do much with a window manager, and the only reasons I went to Gnome/Sawfish from WindowMaker was that the toolbars (with menus, links, etc.) take up very little room and it gets upgraded more frequently. I am extremely protective of my usable screenspace.
One of the upgrades that came along for the ride was the Gnome Terminal. I never used this because I liked ETerm (random backgrounds are kewl), so I may have just never noticed that it had tabs. But it does, and I'm hooked! No more Alt-Tab olympics or moving my hands off the keyboard to find the shell I need, just a Ctrl-PgDn will cycle through my available windows.
Conclusion: I love tabs!