There is real reason for taking Sunni-Shia conflict seriously. It has created long lasting religious war frequently in the past. In fact, even beyond the early divisions, there have been a number of periods of intense Sunni-Shia conflict.
One can see why Sunni Muslims might be worried.
But there are problems with a merely religious analysis of what is going on.
The odd things here are.
I have no conclusions here. Merely that the analysis going on right now seems simplistic. Most commentators seem to assume there is just "Islamic radicalism" with understanding the vast difference between Sunnis and Shia. And even those who understand that do not seem to grasp that Syria is a Sunni country with a minority leadership whose sect is not considered Muslim by either Sunni or Shia ulema.
Andrew Sullivan points to a Slate magazine article on this topic.