Chris Bourdon, Apple's Senior Product Line Manager, just gave an impressive demo of Tiger. I was most impressed with Spotlight. Just this feature alone will make Tiger a must upgrade. It is an incredibly fast search engine, that does the kinds of things Longhorn talks about. It changes the way users use their computers.
I also really like Automater. This is Applescript for the rest of us. It allows users to use the power of scripting with a fast drag-and-drop interface. The H.264 video compression and the ability to use iChat AV to do audio conferences of up to 9 people is also really cool. There is a lot of great stuff in Tiger. Panther, IMHO, was a skippable upgrade, but not Tiger!