I'm very hesitant to answer this, as i know it will bring on angry comments and people telling me i'm wrong, but i'll give it a shot (this is all literally off the top of my head right now, so if you are going to poke holes in it, cut me some slack)

This got a lot bigger than i thought, so strap in!

* The lack of "private" anything. This sounds like a bad idea, but I firmly believe it was a major reason for JS's success. The ability to "monkey patch" anything including built-in functions and other libraries means that everything is extendible. It isn't something i do very often (mucking around with internals of another module/system) but when i do it's really fun and generally solves a problem that otherwise would be unsolvable.

* The debugging. Oh the debugging! It's magnitudes better than anything i've ever used before. And i don't just mean in features (i know that other langs have feature X that JS doesn't have, or can do Y better). I can use multiple debuggers, inspect EVERYTHING, breakpoints, live inline code editing, remote-debugging (on pretty much every mobile device), world-class profiling tools with memory usage, cpu usage, JIT performance, optimizations/deoptimizations, etc... Hell Edge is even getting "time travel debugging" where i can step BACKWARDS in code, edit it in place, then play it forward again! Also, sourcemaps! I can compile python/coffeescript/typescript/javascript to javascript and then minify it and combine multiple files, but when i open the debugger i see my source files, with their full filenames, and the execution is synced statement-by-statement. And they work for CSS too! And I almost forgot about the best part. Since they can be separate files, i can include them in my production build with literally 0 overhead. So if there are any problems with the live build, i can open the debugger and have the full development-mode first-class debugging experience, on the live site, even on a user's PC if i need to. Hell i can even edit the code in-place to see if my fix works! This one is probably one of my favorite features of javascript and it's ecosystem.

* async programming. Yeah, i know other languages have it, but JS is the first time where i would consider it a "first class citizen" Everything is async, it's amazing, and it's to the point that if something isn't async, it's almost a bug. And this combined with the event system and the single-threaded-ness means writing performant code is more "straightforward" than i've experienced in other languages. Combine this with web-workers (or threads in the node ecosystem) and you get the best of both worlds.

* the mix of functional and OOP programming. Functional programming sucks for some things, OOP sucks for others. I feel like in practice JS lets me use the best of both. Yeah, it's not "pure" or "proper", yeah you can use the worst of both, but i love it. You can add the mix of immutable vs mutable in this as well. By having both, it lets me choose which i want to work with for the current problem, even switching in a single project.

* it's fast. JS is pretty fucking fast in the grand scheme of things. Yeah, it's not C, but with typed arrays and some profiling (which JS makes oh so easy!) it's possible to wipe the floor with Python, Ruby, PHP, and can even give Java and Go a run for their money. For such a dynamic language, that's impressive.

* the compilation options. From coffeescript/typescript/flow, to just compiling between js "dialects", and adding non-standard (or extremely new) features to the language is "easy". It took me a little while to get used to being that disconnected from the final output, but once i "let go" of that, i found i loved it. With babel plugins i can add extra tooling, or extra type-checking, or even completely non-standard stuff like JSX or automatic optimizations into the code that i output. Combined with some good tooling i can even change how the code executes based on the "build" i'm generating (for example, i have some babel plugins that optimize react elements to improve first-load speed, but i only run it on staging/production builds because it is pretty verbose (which gets removed when gzipped) and is difficult to debug)

* the tooling. auto-refresh, hot-module replacement, automated testing, integration testing, beautiful output in multiple formats, linting, minifying, compressing, optimizing and more task runners than you'll ever need. The fact that i can write code, save, and have that code hot-replace the code currently running in my page on my PC, tablet, phone, laptop, and VM all at the same time. There is nothing that even comes close to this. At all.

* and i guess finally, npm. The fact that there are 5+ different modules for everything i could ever want. The fact that i can choose to install a 3-line program, or write it myself, or install it first and write it myself later, or vice versa. The fact that i can choose a module optimized for speed, or one for size. The fact that i can get a pure-js bcrypt and a natively-compiled bcrypt with the exact same API and install the native and if that fails fallback to pure-js. The fact that NPM installs are so effortless that i have a project with about 100 direct dependencies (and most likely about 1000 when it's all said and done), and there isn't even a hint of a problem is wonderful (this is a bit of an edge case though, most of the packages installed here are "plugins" like babel plugins, postcss plugins, and i'm purposely avoiding bundled deps kind of for shits-n-giggles.) And no matter how many internet commenters keep telling me i'm wrong, i haven't had any issues with it.

This got a lot bigger than i had intended, but the point is that while JS might not do any one thing very well, it does many things pretty damn well. And the benefits far outweigh the downsides for me.

I'm going to bed for the night, so if you reply don't expect an instant reply, but despite the "standoffish" nature of a lot of this, I want to hear responses.