well, the three differences between DNA and RNA is the 2' oxygen (present in RNA, absent in DNA), DNA uses T while RNA uses U, and DNA is 2x stranded while RNA is 1x stranded (the vast majority of the time).
2x strand leads to stability
mRNA with the cap and poly a+ tail is marked for degredation (research has shown that length of poly A+ tail is proportional to stability outside of the nucleus)
that one 2' oxygen in RNA makes it less stable (why, i don't really know)
does that help at all?