C is not "extremely concise". Lisp, and especially APL both predate C and are more concise. To me, the "killer feature" of C is UNIX, we can't really separate them, C won because UNIX won and vice versa. Other than that it is reasonably concise, reasonably portable and reasonably low level. The language itself doesn't have any "killer feature".

For Python, I think its killer feature, related to the removal of curly braces is that it is a dynamic language (like Perl) that forces you to write somewhat readable code. You have no choice but to indent correctly, and there is a strong "pythonic" ideal way of coding, the complete opposite of Perl's "more than one way to do it".

JavaScript killer feature is obviously its integration in browsers but it is also well designed for this task, with pretty good support for asynchronous programming.

Still, I don't think any of these languages were successful for their "killer features", more because of a combinations of many things. In many cases, they weren't even the first. For example, there are several langages that use indentation as syntax that predate Python. ( https://en.wikipedia.org/wiki/Off-side_rule )