Dustin Browder has posted a new article on the Heroes of the Storm front page describing the leveling and experience gain system in the game. He goes into detail about how the system has changed over the course of development, and also elaborates on why these changes were made.
When we first started, we had individual hero leveling like many games in the genre. We liked how it forced players to split up into different locations, and coming together was a cost and a risk. We also liked how individual experience rewarded players for doing well, and really encouraged players to do their best; knowing that they were falling behind if they started to give away too many victories.
The entire article is worth a read, especially how the current leveling system allows for characters such as Abathur to exist. To check out the full article, click here.
Battle.net - From the Bullpen: Team XP and Leveling