It’s been quite a question: What will DirectX 10 bring that DirectX 9 will not have? For the Multiplayer Crysis, the answer seems to come from Total Crysis (they have a Crytek press information).
Physics and day and night cycle: With DirectX 9, you will not be allowed to break havoc on the trees and other vegetation (it seems that it is not really related to the capacity of DirectX). Time-of-day changes will not be implemented for DirectX 9 servers.
All of this is a matter of development choice and you have to wonder what pressure Microsoft put on Crytek to make such choices… that will help sell Windows Vista (the only OS officially able to run DirectX 10).