I read somewhere in the Quest help that you can leave comments in the game (notes to yourself) but I'm not certain if this can be done in the description text, so far I have not come across any reference to leaving comments in the Quest help file.

Comments would be useful when you have in-game conditions such You see a {here ToyBear:super} big bear

I forgot to say that I was referring to the player interface when entering text into the description as comments.

No replies so it looks like it can't be done.
Would be a useful feature in the next update.

Where exactly do you want the player to be able to leave comments at?

On the main 'game' object there is a checkbox 'Annotations: adds a tab to rooms where authors notes can be made, not seen by player'. So you could make as many notes as you want.
Pixie also came up with a way to check for any outstanding work to be completed.

One way to do that would be to turn annotations on on the Features tab of the game object, then on the Notes tab, list everything outstanding, under the heading "TODO". In the start script of the game, check for any object that has a TODO list:

foreach (o, AllObjects()) {
  if (Instr(o.implementation_notes, "TODO") > 0) {
    msg ("TODO: " + o.name)
  }
}

Every time you play your game you will get a list of rooms that have work outstanding.

Hope this helps

Do you mean you want to be able to leave comments inside a description? It shouldn't be hard to create a do-nothing text processor directive; but I would think it's easier to use the object annotations.

or do you mean like having an in-game journal/note-taking/quest-log/etc mechanic/system?

(wasn't sure what you meant by comments: internal comments for yourself as game maker, an in-game journal/log/note system/mechanic, being able to add comments to Objects/Scripts/etc, or whatever else with comments)

(If you want comments in a description like you can have in text, a little code for the start script:

game.textprocessorcommands = game.textprocessorcommands
comment => {
  game.textprocessorcommandresult = ""
}
dictionary add (game.textprocessorcommands, "#", comment)

Then you can do:

You see a {# here's a comment explaining why 
  the description changed like this
  the comment can extend to multiple lines
 if you want to}{here ToyBear:super} big bear

It basically hides anything between the {# and the next } from the player.

Could also use that for adding whitespace (or newlines and indentation) to complex nested text processor commands, to make them easier to read.

Thanks mrangel and others for your suggestions.