It's not so much that it has X feature or Y widget, but rather that the whole ecosystem feels cohesive. With emacs you sort of cobble together your configurations until you're happy with it, and that's very powerful -- it's why it's so successful. But there's something to be said for having "awesomeness out of the box". It's why Spacemacs was so popular.
I hear you about FOSS. I was reeeeally hesitant to throw my time into yet another closed source clusterfuck. I grew up as a gamedev, and back in 2000 that meant you had to use Visual Studio. All of my VS skills are now completely obsolete. And whenever I ran into VS issues, I couldn't debug it since it was closed source.
Two things convinced me IDEA is probably worthwhile:
1. They're partially open source. https://github.com/JetBrains/intellij-community You can build that from source and run it, so you can at least see how most of the system is architected. You won't be able to extend some of the closed-source plugins, but you get some of the benefits of FOSS.
2. Their tooling is their primary business model. When Microsoft lost their VS monopoly, Microsoft didn't die. JetBrains' sole focus is making the tooling ecosystem work; they're the Adobe of tooling.
>Just type "align" and usually webstorm pops up autocomplete with the right thing
VS Code does both of these things, and is FOSS