Spotlight doesn't just search the file names, it also searches the metadata, including the kind of content, the author, edit history, format, size and other details. Most documents, images and e-mails already contain rich metadata. Spotlight also indexes the content of a document, displaying results based on what appears inside a file or document, not just its title.
Using a separate Spotlight window, it's possible to sort results by date or category. Searching for a particular person in Spotlight will bring up not only their contact information, but also documents they authored or edited, images they e-mailed, messages they wrote and messages sent to them.
This new release date of April 29th is earlier than expected, but Apple feels that it is now ready. You will of course recall that a version of the OS was leaked onto the Internet, so it did seem likely that a release would follow pretty soon afterwards.
"We're shipping Tiger earlier than promised, which we're really excited about. You don't see that happen too often these days with major software releases." - Brian Croll, Apple's director of product marketing.