Status: Implemented for next update.
Currently only "Save as..." exists to save a level. Given that the game now asks far more frequently to save levels, it would be nice to have "Save" as well.
Functionality: If the editor already has a file name for the given level, just overwrite the existing level. If no file name is specified yet, then ask for a file name (as with "Save as...").
Yes, this actually came up in a discussion on IRC the other day. Thanks for putting it here, where I'm less likely to overlook it next time I work on the editor. :)
EDIT: Added for next update. :)