1. no need anymore for kernel bypass mechanisms (too many to name for linux, btw)
2. decoupling of drivers from the kernel
3. instead of waiting for a backport of a new syscall to an LTS kernel, something like that will be just part of your system's stdlib
4. extremely reduced need to reboot a machine
5. driver faults can be recovered from gracefully, just like in Erlang's supervisor scheme
6. seL4's capability system allows for a safer system overall, with less hacks
Seeing how even Intel is introducing a message passing network with growing number of cores in their mainstream CPU designs, we should be over the general objections regarding a message passing design. They call it QMD, and it's something they already have in their Xeon Phi line for some time for obvious reasons, just as countless other general purpose or special purpose chip designs have had for a long time.
I wonder if one could get funding for an effort like Robigalia to reach production quality and have Servo running on it. Given the feature set of Servo, it's a safe bet that the required features would provide a reasonably complete system to replace your Linux machine.
The bigger struggle would be getting hardware vendors to contribute to FreeBSD and Linux drivers to also provide some level of support for a non-C code base. Though, moving up to that abstraction and language level should make it easier to write working hardware drivers.