Miiverse is basically a social network for Nintendo. Users can share comments, achievements, and hand written notes with other users. It is in the system menu of the Wii U and some games have additional social interactions. Games can be suspended to use Miiverse and resumed at the point they left off.
Also, you will be able to combine your 3DS funds with your Wii U funds on a new, unified account. Once the update is live, sign up with the same username on both systems to merge the funds. You will be able to use the funds to buy games on either system.
Hopefully, one day the virtual console titles will be merged as well because I would really like to play Earthbound on my 3DS.
How do you feel about this update? Do you think it was a good idea to merge the two accounts? Discuss in the comments below.