There is a difference, but most won't even realise it.
As example a heli going through smoke. At DX 9 nothing will happen, or a set reaction. In DX10 it can calculate how the smoke/particles will react to the heli. Some minor small detail, but it adds up.
Edit to that:
Just realised you said 'to Crysis'. And I'd have to agree.. If I'm not mistaken the screenshots in the first post are actually rendered in DX9, not 10.
Edit 2:
Other then that; the demo at GC 2007 is the DX9 version of the game. The DX10 gameplay vid has only been shown to some press. Probably DX10 is just a recourse hog and doesn't make all that much of a difference (for Crysis).
One more little thing that you may or may not know:
Crysis/Cryengine2 will be multi threaded, so you finally can actually use your second core or if you are lucky enough to have a quad core you can use the other 3 cores that most of the time are just doing... nothing.
According the Crytek a Dual Core CPU should increase performance by 60% and a Quad Core by 98%.
So that basically means that when the game is heavily loaded it won't let the total frame-rate drop, but certain parts will slow down and the general rendering will continue at normal speed. So lets say your in a tank and blasting everyone to pieces and your PC can't take it, the Particle effects might get slowed down while the normal rendering just continues thus not effecting your frame-rate. But the particles will be shown slower, or with less detail.
Probably the physics will be most effected though.