I like and use both mongo and mysql. Where I think mysql shines is when you've been working on an applicaiton for 5 years, and end up connecting every piece to every other piece in ways you never imagined doing at the start. I think that's a lot trickier to do well in mongo, which favors denormalization since it's a document store. Given that most db's do not actually grow beyond what you can put on a single box, I suspect a lot of people may be avoiding problems they don't have while picking up problems they don't need by blindly switching to mongo.

That said, when you have a problem that mongo solves well, boy howdy is it nice.