Its a bit of a mixed bag. Just getting them set up tends to be pretty iffy; they tend to rely on lots of sockets, which I gather aren't actually implemented by WSL.
In terms of performance once they are up and running, things are actually pretty decent. The biggest bottleneck appears to be I/O, which is kind of a problem for everything WSL. That part makes installing a gig+ desktop package frustrating as it takes way longer than it should.
The funny thing is the types of issues I get with WSL are also had by people using actual Linux. And there's no compositing.
Final answer: Full Linux DEs run as well on Windows today as they ran on Linux in 2007.
Yes, but not everything. They do not implement everything 1:1 so not everything may work. For example, they implemented inotify, but they left away a few things and certain applications crash because they expect certain functionality.
Its special UNIX sockets. Actual TCP/IP sockets work fine. The workarounds for UNIX sockets not working typically involve switching to use TCP/IP sockets instead.
