David Pogue in the New York Times - From Apple, a Tiger to Put in Your Mac [registration required]:

"The most important is Spotlight, which is like Google for your hard drive. As you type into the Spotlight box in your menu bar, a tidy menu instantly drops down. It lists every file, folder, program, e-mail message, address book or calendar entry, photograph, PDF document and even font that contains what you typed, regardless of its name or folder location. This isn't just a fast Find command. It's an enhancement that's so deep, convenient and powerful, it threatens to reduce the 20-year-old Mac/Windows system of nested folders to irrelevance. Why burrow around in folders when you can open any file or program with a couple of keystrokes?

[...] Now, if it weren't for that brilliant Spotlight feature, Tiger wouldn't be as important an upgrade as, say, last year's Panther edition. [...] But with apologies to Mac-bashers everywhere, Spotlight changes everything."