The "marketing fluff" is here [0]. From my understanding, the relatively novel things they're doing at the lowest levels are making it a microkernel, and using a capability-based security model [1].
The former decreases the Ring 0 attack surface, and the latter makes it challenging to cause a confused deputy problem -- you know, the ol' classic "whoops this daemon running as root accidentally allowed a browser tab to read /etc/shadow." Or the time honored problem of a single exploit in one process giving access to all files (and in some cases, all processes' memory and resources) controlled by a given user. Capabilities also make it relatively easy to sandbox userspace code, and to reason about what it has access to. Kinda like containers, but as a core concept rather than tacked on a few decades into development.
Now these concepts aren't new, but they haven't been deployed or supported at the scale Fuchsia may end up at. Which obviously makes it a pretty exciting project in terms of real-world impact. That said, I believe there's been some speculation that part of the motivation for Fuchsia is to avoid the mess that is out-of-tree drivers on Android. So on the one hand, the kernel can be updated more easily, but on the other hand there may in practice be a lot more unpatchable binary blobs floating around doing important things.
For a more academic project that has many of the same security concepts there's seL4 [2], which has the additional bonus of doing some insanely clever formal verification of the kernelspace code [3]. They have formal proofs that the compiled machine code actually implements the specified of the security model correctly, which is the first of its kind AFAIK. They actually have a set of interactive tutorials for the platform [4], which are a great way to get a feel for how userspace works on a security-focused kernel.
As a disclaimer, I'm not associated with either project, and I'm sure my explanations will be ripped to shreds. My information comes purely from following the space in my free time.