I've read on a number of sites that Tiger's primary strength is the substantial rewrite of much of the system core code, simplifying, consolidating, and the like to make future updates more stable and reliable.
That said, Apple had to include something to entice users to purchase it. Spotlight is not eye-candy. It adds major functionality to the OS by indexing not only file names, but file content and associated metadata (info associated with the file...dates, owner, size, etc.). For this to work as advertised requires application vendors to create mdimporters that help the Spotlight engine to index the application's data files. All Apple's apps have the mdimporters already, including for searching the contents of PDFs. The advantage over the long term is that those users who do not carefully hierarchically file their stuff, but just save it anywhere convenient on their hard drives, will be able to more easily find it again later.
Widgets are cute, convenient mini-apps with limited functionality that build into the system some of the functionality we lost from third party apps like Watson. They're a convenient place to jump to check on the latest movies, weather, package shipment status, and the like, without having to drill through websites. I've found them invaluable, and the nicest user enhancement in Tiger.
Whether you buy now, or later, the price will be the same, and you'll need to down the road. The latest talk is that Apple's getting ready to update the commercial DVDs in their Tiger boxes to 10.4.2, establishing a new baseline and reducing the need to immediately commence downloading hundreds of MBs of updates after you install it. For that reason alone, it's probably a good idea to wait a few weeks.
