Pro Code refactoring support
To make sure your code can be easily maintained, you must first be sure to make it clean and tidy. This is the part where WebStorm really helps you. It automatically refactors your code by performing functions on it, such as extraction of variables, moving files, inline variable extraction, etc.
Pro Integrated code quality tools
WebStorm comes bundled with JSHint and JSLint. JSCS, ESLint, and Closure Linter can be installed via npm. They register as inspections and are customizable through IDE settings. They run automatically and will highlight potential issues. Pressing alt+enter on an issue will allow the user to view suggested fixes.
Pro Intelligent code completion
WebStorm has two types of autocompletion: structural completion and word expansion.
Both types of autocompletion work extremely well, have little to no problems and are quite fast even when loading suggestions on the go.
Pro Reduces the amount of repetitive code that has to be written with code snippets
WebStorm includes a feature called Live Templates. Live Templates are predefined code snippets that can include variables. They're intended to increase productivity by reducing the amount of repetitive code that has to be written.
Pro Ability to base hot keys on previously used IDEs
When you install WebStorm, it allows you to choose from other IDE's for it to base it's hot keys on. So if you are switching from another IDE, it makes it a very easy transition and productivity is not adversely affected by having to learn new hot keys.
Pro Built-in terminal/run tools
WebStorm (and really all of the IntelliJ IDEs) support the plugins throughout their plugin ecosystem which leaves you with 100s of tools to handle your automation tasks. There is a wide range of build-related plugins that help you by having pre-defined commands to execute with the click of a button. Out of any other IDE, WebStorm has by far the most coverage when it comes to tools for your development workflow.
Pro Interactive theme (color scheme) editor
Makes adjusting an existing or creating new themes a breeze. Especially due to things like inheritance, as well as easily exporting/distributing/importing the color settings which really only store where on deviates from the defaults (thus the resulting files are very small and relatively human-readable).
Con Not free for commercial use
A paid license is required to use WebStorm for commercial use. The license terms changed in November 2015 and currently require a subscription (per year: $59 personal, $129 for companies). Students can obtain a free non-commercial, educational license good for one year. There is also an option to pay on a monthly basis in addition to perks, such as a fallback commercial license that can be used for free.
Con Plug-in Ecosystem isn't robust
Every framework or extension with any popularity whatsoever will have a plugin for VS Code. Sometimes they'll still support Atom. Only one in twenty will have native support for WebStorm.
If you want support for the latest libraries, you won't get it in WebStorm.
Con Non-native filesystem causes issues
The Java wrapper around the filesystem doesn't actively watch for file changes (by, for example, using the fsevents api on OS X), and as a result can become easily desynchronised from the actual filesystem.
It should be noted though that this is easily remedied by going to File/Settings/System Settings and checking the "Synchronize Files on frame or editor tab activation" option. It's also recommended to more explicitly represent your workflow within WebStorm itself. Most external tools/tasks can be handled with WebStorm. And if it's not in a plug-in, then you can handle it with the File Watchers.
Con Poor usability on option and menu navigation
You need to press a combination of keys or navigate through different menu levels to carry out the two most common things a developer does. For example, to run your code, the default is Shift+F10. To go to definition, you need to click on "Navigation" first. There is no sense to this: being forced to press a combination of keys hundreds of times or navigating through different menus can be a waste of time.
Con Constantly trailing behind on support for its main features
The Jetbrains team do an admirable job attempting to keep up with support for the features they wrap, but they are running an un-winnable race. For example, WebStorm 11.0.2 hangs when trying to debug NodeJS 5.0+ projects (30+ seconds before hitting breakpoint). By the time they fix it, 5.1 will be out and something else will break.