Can someone please explain all this My Chemical Romance "Sellout" and "Mainstream" stuff to me?

Why do some people think that My Chemical Romance have sold out? What does this even mean? And what does being Mainstream mean? Is it bad and why do people think it is bad? I'm so confused! I frigging love MCR sooo much and all this just confuses me because I don't understand why or what people are saying! And...could someone explain how "old" MCR is different from "new" MCR? I know Danger Days is more upbeat and random and crazy, and the old stuff was more depressing and meaningful, but it when people say,"I like the old My Chemical Romance better..." it seems like they are saying that they completely changed. I don't see it! Can someone help explaining this stuff to me?? THANKS! :D