I've got to admit to be being pretty pumped-up about Automator. I'm expecting Spotlight to save my life in much the same way that I'm increasingly dependent on Google Desktop Search on my PC at work. However, Automator seems to be even more exciting since its power appears only limited (to a degree) by your imagination. I really need to start learning AppleScript, particularly since there have been a number of scenarios recently where I've been sure that there has to be a simpler job to automate a repetitive job, and Automator looks like it'll be a great accompaniment to this.
When I think back to it, Panther's only killer feature that I took note of when it was unveiled was Expose. However, after I received my copy of it I found loads of features that I can't do without today (the great implementation of Command-Tab, for example) and therefore would miss if I went back to Jaguar (hell no). The point is that I can see a reasonable number of features in Tiger that excite me (Spotlight, Automator, Mail, Dashboard, Safari RSS and XCode 2, in particular), so if the same experience is true with Tiger as it was with Panther, I should find tons of great stuff in the new OS.