I'll list some of the most obvious and serious ones for you, since you seem incapable of finding this basic information on your own:
- Its type system is horribly broken.
- Its scoping is horribly broken.
- Its comparison operators do not behave sensibly.
- Its prototype-based OO system is impractical, and quite poor compared to other prototype-based languages.
- Its lack of class-based OO leads to awful hacks using its awful prototype-based OO functionality.
- Its lack of support for proper modules and namespacing makes large-scale software development tedious.
- The fact that something as obviously-dumb as semicolon insertion is even conceived of and supported in the first place.
- It's so rife with other bad language features that one of the most widely-respected books about it, Crockford's "JavaScript: The Good Parts", is all about not using large parts of the language.
- Its standard library is extremely limited, and what does exist works quite poorly.
- Its tooling (editors, debuggers, profilers, etc.) is lacking in many respects, and is often entangled within web browsers.
- Its performance is lacking.
- Its community is generally inexperienced and incompetent, and produce a lot of very bad code.
- There's little evidence that things will improve in the future.
While other languages have flaws, none (aside from maybe PHP) have as many utterly stupid, unnecessary and unjustifiable flaws as JavaScript does. And at least these other languages make some real effort to eliminate such flaws, as well. We just don't see that from the JavaScritp community.
