I'll try to summarize the major changes.
-New navigation layout. The navigation menu has changed significantly with new main sections Search, Community, and Profile. Clicking on one of them will bring up a dynamic menu on the left side of the site with additional options.
-New theme. Updated colors and streamlined image and comment display.
-Users can report images as spam, repost, or inappropriate. Mods will be receive notices of these reports and will be able to take action.
-Added thumbnail view for all users. You can enable thumbnail view after logging in and checking the option in the Sort By section near the top of the site.
-Improved search options. The image search page has been simplified. You can still access the full search form by clicking on the "Advanced Search" link. An improved tag search page is also available in the Search section.
-Clicking a spoiler image will now load the thumbnail image for a preview. Clicking it again will load the full image.
-Various action links have been replaced with icons. For example, the comment quote link is now an icon as is the add to favorites link.
-There are a few new options in the profile editor. You can disable spoiler warnings if those annoy you and you choose to always display all image metadata (so you don't have to click "See all info").
Much of the code for these changes was done by ADantes. Thanks goes to him for all his work.

As always with big changes, there may be lingering bugs. If you find anything wrong, please post here or PM one of the mods.