Compatibility is definitely the one big thing SerenityOS is missing. A nontrivial effort for sure, but if it can run Linux and Windows binaries, you are going to see much more adoption.
But then you've basically just made ReactOS again?
ReactOS is a kernel-level implementation of Windows NT. Serenity is a Unix-like.
For the kind of people who read Hacker News, this does sound like a significant distinction. For everyone else it's a mere implementation detail.