We already knew that the Borderlands 2 Season Pass wouldn't include the Mechromancer. The extra character is a bonus for those who pre-ordered the game, but will cost $10 otherwise. Apparently Gearbox has plenty of DLC in the pipe, some of which also won't be included in the Season Pass.
"We will have other, non-season pass related DLC. The additional character (Mechromancer) is one kind of example," Gearbox CEO Randy Pitchford said in a Kotaku Q&A. "The Borderlands 2 team is fired up at the moment. Customer love for the game is driving us wild - making us want to do more and more stuff. So, expect that there will be even more beyond the big Campaign DLC in the Season pass and the add'l character. We'll announce more stuff as soon as we are able to be confident in the commitment of the details we share."
He says the Season Pass content is bigger, and compares it to the General Knoxx DLC from the first game. He says the team is very far along in developing the first and second packs, while the third is in earlier stages and the fourth is in the pre-concept stages. It seems like the Season Pass still assures you get all of the substantial content, but you may have to shell out a little more if you want every possible piece of the Borderlands pie.