There is a nice story on Marvel vs Capcom 3 over at Eurogamer. It features some interview quotes by the game's producer, Ryoto Niitsuma, and is a pretty interesting read.
One part that really caught my attention is the following quote. Hit the link to read the whole article.
"Everyone had a Level 3 initially," Niitsuma says, "but because we wanted to continue the tradition of a purposely unbalanced roster, we intentionally left a few characters out. They don't all have an over-the-top style, and for these characters, we balanced it out by adding in something else."