What functionality is missing? What is unsatisfying with the current situation?

Whenever you use an article link (aka mention), it shows a long ID that looks like (person:XXXXXXX-XXXX-XXXX-XXXX-XXXXXXXX). This can make the editor a bit messy to look through, since you'll be scanning past long code IDs whenever a mention is used.  

How does this feature request address the current situation?

A toggle to hide these IDs would make things much cleaner to look at, especially for the casual user side of things. You'll be able to clearly read just the text, without a bunch of strange codes mixed in.   The Euclid editor already has a "Spellcheck" toggle, so a per-textbox "Mention ID" toggle could work similarly. Or it could be page-wide, like the "Quick Switch Edit Mode" toggle at the top of every article's page that hides the Edit Article icons.  

What are other uses for this feature request?

When users want to see the article IDs, maybe for copy-pasting the mention, they can toggle it back on.