To be honest I don't know the exact anwser because I've never used PS v.6. Though I do believe like Ashley has stated have more options in the filters and tools (brushes etc). Also there are more layer tools and options.
(This is a copy and paste)
* The ability to customize and tidy up Photoshop's multitude of palettes into savable Workspaces.
* Similarly, Tool Presets can now be saved.
* A File Browser, carried across from Adobe's low price (but highly useful) Photoshop Elements, lets users search for graphics by name or date or keyword.
* A greatly revised Brushes palette includes features to approximate real-world painting, reminiscent of (though still a pale imitation of) Corel/Procreate Painter.
* Two new tools, Healing Brush and Patch Tool, that work magic in erasing wrinkles and other minor flaws from photos.
* Auto Color adjustment, which simplifies correcting color casts in scans or digital photos.
* A spelling checker. You won't use Photoshop to replace your word processor, but now you'll have no excuse for making spelling mistakes when you add text to images.
(also more text affects)
Check out:
http://wdvl.internet.com/Reviews/Graphics/Photoshop7/
THey have some other features described in more detail.