I upgraded one of my sites to 4.2.1 and my RSS feeds stopped validating. It complained that there was a » in the header and this was invalid. I did some digging on the wordpress site and they said it was down to a particular theme (not suffusion I might add).
My other sites, running 4.2.0 were OK, so I deinstalled 4.2.1 and reinstalled 4.2.0 and everything is OK now.
I thought you should know you can track the issue down and fix it.