The difference is almost uncomparable and incomprehensible! lol. More, more, and more real-time events with so almost every visible thing having its own AI. For instance, you don't walk through trees (like older DirectX), and with the newer directX's you might see only some large piece of the tree move, well nothing like that. If you brush into a leaf or two, just that one or two pieces you directly bumped will act.
DirectX 10 will act with GREATLY enhanced shading, texturing, lighting, and depth effect enhancements. From the buzz, there's going to be no comparison between 9 and 10.
Check this out. It's been deemed these images aren't precisely how DX10 would compare (which when you read through and look at the pics they tell you this) to 9 but its very revelant and close to what DX10 will be able to handle. Most all DX9 code has been rewritten for an all-new DirectX 10!
And this is only the beginning!