Major changes include a giant update of the Map Editor, the addition of a new "prestige" system for co-op mode that adds new depth of gameplay, new achievements for single-player campaigns, the addition of two new announcers (White-Ra and Stone), and more!
New custom map mode to let people build campaigns, all of the recently posted editor stuff, achievements, announcers, lobby options, bug fixes, co-op balance and design pass, enormous new co-op features :D
Here is a list of all of the co-op prestige talents:
Sounds like there is more stuff coming for 5.0 soon too, e.g. talk of a new co-op hero release and blizzard devs on episode 100 of the pylon show to unveil and discuss some stuff.