You can now post new messages through the PC.
EDIT: You can also follow people now from the PC.
EDIT 2:
EDIT 3:
EDIT 4:
EDIT: You can also follow people now from the PC.
EDIT 2:
The update looks to be pretty substantial beyond just posting from the web. You can now also follow people from the web app, as well as view friends/following lists, update profile details, and edit general Miiverse settings (i.e. who can comment on your posts). All that's left to add seems to be Private Messaging.
EDIT 3:
(You now can) View any Miiverse post, even if you don't have a Nintendo Network ID or are not signed in. As long as you have the URL for a post, you can open it.
EDIT 4:
The restriction between making new posts has been reduced from five minutes to three.