http://us.battle.net/heroes/en/blog/19909253
For folks like myself who barely play HotS and have zero intention of grinding out XP+gold, now is the best time to level up (account as well as individual hero levels) due to this bonus. I had basically stopped short of account level 10 (where one of the rewards is a 7-day stim pack) in hopes there would be some kind of buff like this.
All bonuses stack including the one for playing with a friend. It’s also just best to level by playing against the AI at either the beginner or recruit difficulty (basically, whichever difficulty level where you can complete games in around 10 minutes).
The most XP I’ve gotten so far in a game (when played with at least one friend) is around 400,000 playing the recruit AI. It’s easy to get any hero quickly to level 6 this way as well (which is where the first set of portraits tend to unlock).
