Note that our plugin system is a major departure from how SMF works. No SMF mod will work without being rewritten, since the plugin manager does not permit file edits of any kind.
The short synopsis off the top of my head though, in no particular order...
Added
Auto embedding
Gallery
Posting and PM drafts
Flexible skin support using LESS syntax, allowing for convenient support for gradients and other CSS3 images without worrying about browser-specific syntax
bbc/WYSIWYG editor available in the quick reply area
Thoughts system
Liked posts
Per-topic feeds
Bad Behaviour integrated by default
IPv6 support
Template macros such as <we:title> and <we:cat> instead of using standard markup, allowing for themes to set their own markup entirely for such things without mods having to be updated (cf. all the issues relating to RC2/3 and RC3/4 mod compatibility)
JavaScript files are minified by default and JS is managed so that JS files are all loaded late in the process and not in the head - meaning that things look like they move more smoothly
Merge double posts
User and action menus in posts
An automatic quote splitter - use the quote function to copy a large post, then use shift-enter to insert a break in the quote where the quote tag gets closed before the cursor, a new opening tag after the cursor so you can easily reply to large posts
Spoiler, footnotes, more bbc added
Gravatar support
Using a board as a blog
A revised plugin manager that relies on a stronger hook system, allowing for plugins to cleanly interact and install easily
Changed
Default theme - radically overhauled and not using large images etc.
Revamped admin home page, improved admin search
Completely rewritten CAPTCHA
Improved page navigation, and previous/next topic navigation
Revamped theme layers system to allow for strong but arbitrary nesting of containers
Some elements of code have been rewritten to OOP style
-- though a massive amount of stuff had to happen to make these changes!
Removed
Any language support that isn't UTF-8 only
PostgreSQL and SQLite support
Pre PHP 5.2 support
move, glow, shadow bbc removed
SMF's package manager