The XP from mobs was placed onto the bosses instead. This was done to encourage people to focus on killing bosses for XP and not just grinding a hunk of mobs before resetting. It was also adjusted for the XP squish.
In the end, you get around the same xp as before, but now you can skip enemies along the routes to the bosses(where applicable) and come out on top. It's a pretty nice change all around.
