AX2012r2 with DC running Windows 2003 Mode

Hi,

I can see in the system requirements that AX2012 needs Windows 2008 mode for the domain requirement:

"The domain controller must run in Windows 2008 mode or later."

However our infrastructure is still running 2003 mode. What problems could it cause to run AX with a 2003 mode? Why is 2008 needed?

Thanks!