DataCenter the name itself implies what would be the probable use of this edition. Parallel Data warehouse is an interesting edition which is called an appliance-based solution i.e. in simple terms you need to buy hardware along with this version. I discussed in my previous post about this concept. Pricing of both these editions seems to be very similar, in fact its $57,498 for both of these editions for per processor quote which can be read from this page.
One other interesting difference between Enterprise and Standard edition is that Master Data Services and Powerpivot for Sharepoint would be available only in Enterprise edition and not Standard Edition. On the top of it, Master Data Services would work only on 64-bit editions. Also Sharepoint Server 2010 would work only on 64-bit hardware and architecture. In summary if one carefully thinks, it's better to use 64-bit hardware and OS architecture if you are planning to buy Enterprise Edition of SQL Server 2008 R2 as most of the features that it offers are available only on 64-bit architecture and not x86.
To get a higher level view of the differences, read this datasheet which gives a clear idea of the same.