I upgraded one of the Macs in my house to Tiger last Saturday and it is really great. My favorite thing at the moment is the RSS reader in Safari. I have a folder full of RSS feeds (blogs, news, etc.) in my bookmark bar. Safari puts a number in the title to this folder that tells me how many articles have been added since the last time I looked at them. It is really nice and much easier to see what’s new in lots of different areas. Much nicer that using Firefox or an external RSS reader which I’ve done in the past.
Dashboard, Spotlight, and lots of other things are also really cool about 10.4 but Safari RSS seems to be the thing that I am going to get the most use out of.