ReactOS is an implementation of Windows Server 2003, still in alpha stage, and very interesting.
But it is no threat to Windows. None at all. Especially compared with Linux.
Here's why. Sponsors.
Linux has big corporate sponsors behind it, starting with IBM and Google, companies which depend on the software, which run it in scaled installations, and make money off it.
React can't even think of going there. (Unless, say, HP endorsed it, and that's not going to happen.)
There's a second reason React is going nowhere. It's server code. Servers need support, hands-on support, for updates, for changes, for bug fixes. When your Web page reads "long term plans" with a big n/a under it, as that of ReactOS does, I'm not trusting my business to it, are you?
If we had an open source Windows desktop, compatible with XP, you might have something, but not with a server. Especially if it were free as in beer.
In rhapsodizing about the wonders of ReactOS John Dvorak praises its drivers and moans about the Windows registry. Good points, but these are essentially desktop functions, not server functions.
ReactOS, with sponsorship, might make it in some home offices and small businesses, but it will never get past that niche, and like Windows it will need support. It won't be free.
Which brings up a final point. Bugs. Is React going to copy Windows' bugs? Of course not, but is it going to be vulnerable to the same sorts of bugs? Could be.
If I'm going to look at a new desktop OS for a threat to Microsoft Windows, I'll look at the iPhone.