Steam – How to backup the Borderlands Character

I have the Xbox version, but the maximum level without DLC should be 58. With "The Secret Armory of General Knoxx" the maximum is 69 (it goes to eleven, har har).

"The Zombie Island of Dr. Ned" doesn't affect the maximum level. "Mad Moxxi's Underdome Riot" doesn't either, but it does provide 2 extra skill points you only normally get by ranking up. One for beating the "lesser challenge" (only 5 rounds) in each of the 3 stadiums during playthrough #1, and the other for doing the same in playthrough #2.

The level cap for all characters was increased by 8 with the release of patch 1.4.1. This makes a total of 69 skill points to earn, though the four gained from DLC's 2 and 4 will still require those add-ons (as they are mission rewards). (source)

You can only get experience for a mission once per playthrough. If you completed the mission by yourself and then replayed it in co-op, you would not get the experience a second time. If you complete a mission in co-op, it will count towards your single player game for all players involved, if they have not already completed it.

It is also possible that you are not eligible for the mission, which means you have not completed the prior main story-line missions and are not far enough ahead in the game to get that mission by yourself. You will not get the rewards when another player turns it in. If this is the case, it will say so next to the missions and will not track progress for the mission.