There are some meta-commands that are clearly required in a game system, such as HELP. Others I am not so sure about.
Suppose my game runs in a web page…
Is there any point in having a RESTART command when there is a Reload/Refresh button on the tool bar?
Does a QUIT command make any sense? What would it even do?
What about a CLEAR command to clear the screen? Is that something anyone would ever use?
These are not particular hard to implement, but is there really any point?
Restart can be distinct from Refresh where the game auto-loads your most recent save (or auto-save). This is itself a very useful default behaviour for a browser-based story.
An explicit RESTART command is useful, as @mirality wrote. Refreshing the page can often happen accidentally or automatically in a mobile phone. Thus you need a separate mechanism for restarting.
I don’t think QUIT makes sense on a web page.
CLEAR could be useful if the author never clears the screen, since very large web pages can bog down browser performance, especially on mobile phones. But it’s probably better to leave this in the hands of the author, or have old content scroll off the top of the screen.
UNDO is expected nowadays, I think.
I’ll just throw in a suggestion for PRONOUNS, which is occasionally handy even outside of debugging.
VERSION should be provided by default by the game system, because you can’t add it later when you realize you needed it.
Thanks.
What is PRONOUNS?
If a story is part of a collection, then QUIT could perhaps trigger navigation back to the index of other stories.
At least in Inform, and possibly others, once you’ve referred to a specific object in a command then you can later refer to it using its pronouns – e.g. examine trophy followed by take it or say hello to Jeff followed by ask him about the MacGuffin.
On occasion you might lose track of what pronoun is currently referring to what object, so you can ask.
If you don’t put them in then people will complain about their absence; if you do then nobody will notice. So you might as well include them.
QUIT and RESTART are synonymous in my book and I’d habitually use them rather than reloading the page from the toolbar (Adventuron runs in the browser and includes these meta-commands).
The usefulness of commands will depend on the environment your game is running in. Here’s what I think for a game running in a web browser.
QUIT indeed doesn’t make any sense, you can safely leave it out.
It might depend on the game, but I think a RESTART command often remains useful even if the game also restarts if you reload the page. On a side note, I think reloading the page shouldn’t restart the game but rather resume where you left off.
PRONOUNS doesn’t feel important to me as a player (it might be useful for debugging but I’ve never felt the need for it).
I didn’t know of the CLEAR command so I wouldn’t miss it.
I recently added VERSION to my own game after beta-testers tried it, but I’ve never felt a need for it (though if you’re distributing the game in way that can leave several versions in the wild it starts making much more sense).
I think UNDO is considered a requirement and players will probably be quite cross if it’s missing.
AGAIN (G) can be quite useful (depending on the game) and would probably be expected to work.
OOPS (O I think) is not something I’ve ever used or implemented in my games but I think some people still use it.
ABOUT / INFO / CREDITS should also be recognized but it’s fine if they all show the same message as HELP.
HINT should at minimum apologize for the lack of hints.
SAVE [slot] / RESTORE [slot] if your game supports it.
As far as I’m concerned the verbosity commands (VERBOSE / BRIEF / SUPERBRIEF) belong to a bygone era and can be omitted but I wouldn’t be surprised if others disagree.
EXITS is nice to have but I don’t think people will get up in arms if it’s missing.
SCORE (and maybe FULLSCORE) if your game is scored. NOTIFY [ON/OFF] I’ve never felt the need for.
A TRANSCRIPT / DUMP command for getting a transcript if very useful.
I agree about G, I use that a lot myself. OOPS turns up in transcripts every now and then.
An example where PRONOUNS is useful: I habitually use it/them/him/her while playing, as it is often faster to type than the full noun. Suppose I see a pair of glasses on a bedside table. I attempt to GET THEM, but the game replies that “They are fixed in place”. Perhaps they are, but it’s more likely that the author just forgot to mark the noun as plural, in which case GET IT would now do what I want. But it’s also possible that the author forgot to tell the game system to set the pronouns from the glasses—and in that case, GET IT might do anything, including picking up the booby-trapped scorpion next to the pair of glasses. So I would type PRONOUNS just to ensure that the game and I are on the same page.
I kind of agree with @nilsf about VERBOSE, BRIEF, and SUPERBRIEF, and there’s indeed no out-of-the-box support for them in Dialog. On the other hand, I can imagine that they are useful to people who play with a screen reader.
In my games I always print the value of a pronoun in brackets before replying to a command that includes one; that may color my lessened need for PRONOUNS. Now that I consider it perhaps I should be less verbose, I prefer it but perhaps other players find it a tad spammy.
I guess I’d just rely on UNDO in this situation. 
That’s a very good point. 