I was already thinking Tiger would be pretty cool with the new Spotlight feature, and the Dashboard looks like it will be nice eye candy. But this article went into a lot more detail, and there are some serious changes underneath.
Spotlight is just the initial payoff from some deep improvements in the filesystem. Tiger lets you set arbitrary name-value pairs on files, and there are kernel hooks that trigger updates to indexes of this metadata whenever a file is touched. This is what makes Spotlight search so fast. And this also means that finally the Finder will respond instantly whenever files are created or moved. And i'm pretty sure it means that we can look forward to never having to wait to add up the sizes of folders again — it should be possible to make a map of disk usage instantly instead of waiting ages for du or OmniDiskSweeper to scan the whole disk. It probably means that backups can be done much more efficiently, too. There will be so many things that can be built on this metadata — faceted classification, here we come...
Ah, at last, a modern filesystem! Not quite as futuristic as the Be filesystem, but definitely getting there.
Tiger also plans to overhaul the mess of file type conventions (extensions, type/creator codes, MIME types, and so on) with a file type standard to rule them all: a hierarchy of types that allows a file type to inherit from more than one parent, and that separates the hierarchy of namespace politics from the hierarchy of subtypes.
The graphics architecture is changing, too, in a big way. The big step for Quartz was to paint into a framebuffer in RAM and then let the graphics card composit the windows onto the screen. But painting windows takes a lot of CPU time, and the drawing routines were quite a bit slower than QuickDraw was. Tiger has optimized the heck out of those drawing routines, speeding some of them up by factors of 2 or 5 or 10. But Tiger prepares for an even bigger step — the plan is to move all the drawing to the graphics card as well. All the icons and widgets and glyphs will get turned into texture maps and the whole screen will just be one big OpenGL scene.
Whew. There's a lot going on in there.