(This is a modified version of a blog post from the Cats are Liquid Dev Blog.)
Let's start with the two biggest additions:
Speedrun mode
You can do world specific speedruns, or speedrun the entire game from beginning to end. The game gives you a timer and (if you've completed a speedrun of this specific type before) a live comparison to your previous time. Custom packs are also supported, provided that creator has marked it as supporting speedruns.
Challenges
Each official world now has a challenge connected to it!
The difficulty of these challenges range from quite easy, to turning the room you're in into a giant puzzle where you need to think very carefully about your next move.
Challenges run automagically in the background, so you don't need to do anything special to start them. You'll also get an achievement when you complete a challenge!
New stuff for the editor
There's liquid grates (kinda like those cloud grates, but they require you to be in liquid form to pass through), button prophecies (you can use a prophecy to activate a button, or multiple buttons), Companion activated prophecies, buttons that can be unpressed, and timed buttons. Oh, and some additional tweaks here and there too.
Skippable text
Text prophecies that take control away from you can now be skipped! (Do note that this is just the text, the game doesn't just magically teleport you to the end of the interaction.) This is also supported in custom packs, provided that the creator of those has marked the prophecies as skippable. Skippable text is on by default in new packs.
A bunch of other stuff
The changelog file I have is 175 lines long. That's pretty long.