which-key being integrated everywhere has made discovery the easiest of ANY editor/IDE I've used. Imagine if there was a consistent place where the menu would pop up if you faltered any key combination. If I'm trying to go a git command in Android Studio (InteliJ) and I forget the command, I don't get part way through and see a popup list of contextual actions. I'm left to move my mouse to the right place, click, and then hope to find my git action in the menu I picked.
More common is the unified way that helm makes dealing with files (you can find similar things in Sublime / Atom / Vim distributions using Unite (or whatever they used these days)). Even though the better editors have this, it's not universal and still a differentiating factor for usability and consistency.
Spacemacs integrates Magit by default. Magit is the first version control interface complete enough that doesn't make me immediately nope out into a terminal for proper git commands.
In my primary projects, I need to edit XML, Clojure, YAML, SQL, Java, Makefiles, and Python files. Spacemacs handles syntax highlighting, autocomplete, refactoring, repl connection and evaluation (where available), project management (makefiles and maven support), and documentation lookups.
