What do I lose by running Exchange 2010 in a Windows 2003 Active Directory Domain?

I know that Exchange 2010 can be run in a Windows 2003 domain but I'd like to run it in Windows 2008. I need to justify it though. Will I lose anything in Exchange 2010 if I only use it in a Windows 2003 active directory?

(I will have Exchange 2010 running on Windows 2008 R2 64bit.)

Thanks.

update: I'm not asking about domain controllers or running Exchange on a DC. I'm asking if my Active Directory is only at the level of 2003 and I run Exchange (on my 2008 server) in that domain, what do I lose? Or, in other words, if I put Exchange 2010 in a 2008 R2 domain, what advantage do I have in Exchange that I wouldn't have had if I had only used a Windows 2003 domain.

There aren't really Exchange features that are "unlocked" by having a higher domain or forest functional level. (You can have a look at the formal support policy from Microsoft for domain and forest functional levels).

Exchange doesn't care at all about your Active Directory version, as long as all domain controllers are at least Windows Server 2003 and the domain and forest functional levels are also Windows Server 2003; if you don't have Windows 2000 DCs around, Exchange is not going to ask for anything more.

If you have lots of users you might benefit from having 64-bit domain controllers (which anyway you could also create using Windows Server 2003 x64); but there really isn't anything more than that.