I'm guessing you mean “no parallelism”, right? Because JavaScript has (admittedly horrible) concurrency support. Otherwise what's this “callback hell” some people complain about?
Parallelism I see as a hardware feature that can make programs faster.
I thought about the "callback hell", this is a bit more like coroutines, a quite limited form of concurrency (like non-preemptive threads). In this sense Js is very limited in what programming techniques can be taught.