Just what is says on the box - new spoiler tags! Not been happy with LJ, lately, but this has definitely improved my opinion some. From the news post:
Spoiler Tags: Plots no longer ruined! How many times have you visited your favorite TV show fan community only to find someone has carelessly posted an unknown plot detail without the courtesy of a spoiler alert? Release 90 is here to help: You can now add spoilers to an entry through use of the [Spoiler (click to open)] tag. Though similar to an lj-cut, an lj-spoiler does not require a new page to load when clicked. Clicking the link will expand the hidden text inline. Plus, it works on both entries and comments. You can customize the text by using the format [”Custom]. There is also a new button for lj-spoiler in the Rich Text Editor that can be used in the same way the lj-cut button is used.
Works in comments and posts, and it's not a cut so you don't have to load a separate post for it. Very happy with it, especially since it has so many other applications (i.e. warnings).
I really hate spoilers, so please spread the word about this as fast as possible, get people to start using it, and safe the rest of us a lot of spoilers, triggers, and other problems that can now be solved with a single tag instead of needing lines of HTML coding to deal with.