Tapbacks & full-screen effects
Think of Facebook post reactions... but for your iMessages. Instead of writing a reply to a message you just got, you can respond even faster with a quick thumbs-up, a thumbs-down, or a HA-HA, among other options, that'll appear right on top of the message.
Conversely, if you want to make your message big and elaborate, the Messages app will come with full-screen visual effects. As demonstrated in the keynote presentation, saying things like "Happy New Year!" will arrive with a giant fireworks display across the screen.
App Store for iMessage
Apple is finally opening iMessage up to developers beyond third-party keyboards. This will allow you to further customize what you can send and receive in the app, like stickers, payments, and apparently, tons of other cool stuff that developers haven't even thought of yet.