1. Has a strong copyleft license – GNU General Public License (GPL). So there can be no proprietary derivatives. Also, proprietary kernel modules cannot be linked to the Linux kernel.
2. C++ is forbidden in the Linux kernel. So kernel modules cannot be written in C++.
3. Monolithic kernel – not hybrid or microkernel. That means less reliability and security.
What could be better
1. Linux should be allowed to be linked to non-GPL kernel modules as long as they are GPL-compatible (including but not limited to MIT and three-clause BSD. Apache and CDDL?). I am not saying that proprietary kernel modules should be allowed to be linked. The copyleft should be softened or minimized (the only concern about softening the copyleft is that it might make loopholes).
2. Nothing really.
3. Nothing really.
The good news
1. Not as strong copyleft as GNU Affero General Publuc License (AGPL). Even AGPL is not as strong copyleft as Sybase Open Watcom Public License. Kernel modules can be dual-licensed GPL with any compatible license. I think the copyleft of Linux has an exception for binary firmware blobs. I think proprietary software can be bundled on top of GNU/Linux as long as the underlying system is open source. I think proprietary kernel modules can be linked for personal use.
2. Nothing really.
3. Linux is becoming less monolithic and more like a microkernel. Linux is getting the modular capability. So it’s no longer purely monolithic, but a modular monolithic.
General talk 
1:I strongly say “YES” for that Linux is always protected by copyleft, NOT copyright.
2: I think Rust or Go is good for Linux. Linux contains Assembly language otherwise it cannot boot.
3: GNU Hurd is the replacement.
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The vanilla Linux contains non-libre drivers and non-libre firmware. The word “free” can refer to money, so it’s better to use French or Spanish word “libre”. The 100% libre replacement are GNU Hurd+Mach or Linux-libre, https://www.fsfla.org/ikiwiki/selibre/linux-libre/index.en.html . Open source initiative has recommended sybase open watcom public license. On the other hand, free software foundation said the license is *not* libre, since the copyleft is too strong. The weaker copyleft license —AGPL— *is* libre. Parabola’s blacklisted Oracle VM VirtualBox https://git.parabola.nu/abslibre/blacklist.git/tree/blacklist.txt because VirtualBox requires sybase open watcom.
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