Mac OS X 10.4 "Tiger" is supposed to contain over 200 new features. For the most part your average user probably isn't going to notice very many of them: the general look-and-feel of the OS is very similar to the previous version. That's a good thing mind—we don't want to do a lot of relearning every upgrade. With Tiger it's the refinements that matter. Here are several that deserve exposure...
Command-Space for Spotlight searches
From anywhere, in any app, hit Command-Space together then start typing: Spotlight rapidly shows you the results of your search including a "top hit". Use the arrow keys to navigate down to it then hit Return and your application launches/ file opens:
Control-Command-D for pop-up definitions
Surfing the web and come across a word with an unclear meaning or suspect spelling? Mouseover the word in question and simultaneously hit Control-Command-D for a pop-up definition:
Unfortunately, this doesn't work in all apps, such as Microsoft Word.
Annotate your PDF documents
They used to be read-only: not any more! PDF is a common way to archive documents (e.g. a scanned health insurance application). Likewise, it is a fairly reliable cross-platform "standard" for sharing documents with others. What's been missing (unless you buy Acrobat) is the option to mark up such a document, drawing attention to a particular section for example. Preview 3 includes text and oval annotation tools and allows you to save these annotations into a PDF. Although you can add a bookmark to them, it doesn't appear that you can go back later and remove the annotations:
I'm a writer, and it's incredible how useful the on-demand dictionary and thesaurus are. That feature wasn't even on my radar when I decided to upgrade, but it's the one I get the most out of.