First and foremost is the introduction of a new Dark Mode, which brings a dark theme throughout the operating system. It is currently possible to get a dark Menu bar and Dock in macOS High Sierra, but macOS Mojave extends the darker colors to all system apps and the overall UI. It's a nice touch, and one that will likely be appreciated by many.
In order to cutdown on desktop clutter (you know; photos, documents, spreadsheets, etc. strewn all over), Apple has introduced Desktop stacks. Stacks automatically groups similar file types together to bring some order to your messy desktop. You can expand a stack to see what's inside or "scrub" through all of the items with your cursor without having to open them up individually.
New app additions to the macOS fold include Apple News, Home, Stocks and Voice Memos, which have all previously been featured on iOS. There's really nothing more to add on that front other than that those of you familiar with these apps on the iPhone and iPad will be right at home with the "desktop" counterparts.
The App Store is getting a much-needed overhaul (as we previously reported) and takes its design cues from the iOS version (which was redesigned in iOS 11). Finder adds a new Gallery View, which allows you to scroll through oversized previews of your files. Quick Actions allow you to make quick edits to files from within Finder, and you'll now be able to see full file metadata in the Preview pane. There have also been some improvements made to taking screenshots in macOS Mojave. You can set a timer for when you want a capture to take place, customize the default location for saved files along with a number of other fine-tuning options. Finally, Group FaceTime is also making an appearance in macOS Mojave.
However, one of the biggest additions to the macOS experience that was announced today isn't yet available for developers. Apple's Craig Federighi announced that developers will be able to port their existing iOS apps over to macOS. macOS apps use the AppKit framework and iOS apps use UIKit. Apple is integrating UIKit framework into the macOS to enable this cross-platform compatibility. Apple is reserving this functionality for itself at the moment (with the first ported apps being the aforementioned Apple News, Home, Stocks and Voice Memos).
Developers will be able to port their iOS apps to macOS starting in 2019.