First of all, and most importantly: The Steam page says next to nothing about new content, despite Ultra Deluxe having literally double the original game's content. Now, onto the full review:
From the ashes of depravity rises the phoenix of quality. How else to describe The Stanley Parable: Ultra Deluxe?
The original game was full of lengthy ruminations on the nature of choice in video games, and of course Ultra Deluxe is as well. But this one also has lengthy ruminations on the nature of sequels to video games, lengthy ruminations on the nature of game development, and even lengthy ruminations on the nature of... buckets.
It would be hard to choose my favorite ending, so I'll choose two: My favorite new ending is the Silly Birds Ending, and my favorite returning ending is, of course, the Broom Closet Ending.