Syntax-wise, the CLR's threading is much more flexible -- you can have your workers talk to each other directly when that's the most convenient way of doing things, instead of the Web Workers model where everything has to pass through the main thread. The CLR also supports more than two number types, which is useful in a number of situations, such as art programs, where most if not all of your actions are going to be against smaller types. Web Assembly fixes some of these problems, like providing reasonable number types, but at the same time, ignores other flaws that still exist, like a lack of vector types, which is something nearly every modern processor has and uses for improved performance.
The problem with Javascript, and its various extensions and adjuncts is that it still suffers from the same walled-off mindset that it has for the past fifteen years. Numbers are numbers and you're crazy if you want anything other than these beautiful 64 bit floats, a compact, easy to parse bytecode format is an affront to the open web, even if the alternative is a hack that is just as unreadable, and threading will just lead to programs crashing. Javascript may be fine if you're delivering a web page, but the tools it gives make it woefully unusable for near anything else.
