Here's the official changelog, anyway:
DMs: The media upload buttons in the DM composer will now dynamically show and hide on smaller screens to give you more space for composing messages.
- Quote Tweets: “Retweets with comments” are now known as Quote Tweets! This change has been rolled out globally for Web. Look for Quote Tweets between Retweets and Likes on the Tweet detail screen, and the Quote Tweet option in the Retweet dropdown menu.
Composer: Clicking a conversation card will now show a preview of the card within the composer after opening.
DM Drawer: Opening a link for a Tweet or Event from a DM on the Messages screen will now navigate you to the correct page, and open the DM Drawer to the conversation you opened the link from.
Emoji Picker: The Emoji Picker’s performance should now be a lot smoother and snappier when opening and scrolling through categories.
- Tweet Detail: All timestamps on the Tweet Detail screen will now link to the Tweet’s URL location.
Plus a truck load of bug fixes that I won't list.
PS. As a reminder, this application now trumps third party options like Tweetium UWP because of recent Twitter changes which knocked push notifications on the head. Along these lines, see also my (older) general round-up of ways to access Twitter under Windows 10 Mobile.