Obviously I'm not talking about current implementation, but what it needs on the basic, theoretical level.  
In PoW, you only need the chain of proofs, without transactions, which lead you from genesis to the latest balance sheet.
In NXT, you need account balances to verify blocks, so you need full blocks, you just don't need to store them (but someone has to!).  
You can update genesis in newer versions of the client, but that's centralization really...