R2 is a licensing thing. It has nothing to do with kernel changes or service packs or other such things.
Remember 2003? And then 2003 R2? 2003 R2 did NOT have any kernel changes. 2003 R2 was basically a bunch of add-ons, SOME of which were freely available from Microsoft, some were not. It did introduce some new technologies/advanced some existing ones and bump the schema up a version, but the KERNEL was the same.
Server 2008 R2 is radically different from Server 2008. There are MANY kernel changes, especially in the area of power management (core parking for one) and the ability to handle (with the appropriate version) 256 Processor Cores.
R2 provides a way of saying "Your CALs are still good". With R2, you buy the SAME CALs (the CALs don't need to be upgraded) that you did with the original version.
(FYI, I saw a presentation on 2003 Server by a Microsoft Rep who challenged the thought that Windows 7 was Vista R2 and upon hitting him with the logic based on kernel similarities, he explained why R2 exists).