Subject: Why do self-sufficent 'ribozymes' use RNA and not DNA?

To someone more knowledgeable than I:

I've read a good deal about the origin of life.  One oft-cited theory 
has to do with self-replicating strands of RNA.  My question is, why 
RNA?  I know it's simpler and more likely to come together in some 
sense than, say, a protein enzyme, but why is RNA favored over, say, 
something more like DNA?

Is there something about ribose over deoxyribose, or uracil over 
thymine?  Or can a ribozyme catalyst be made out of DNA as well?

Re: Why do self-sufficent 'ribozymes' use RNA and not DNA?