In the section titled "The DM is the Boss", Moldvay says:
"The DM decides how these rules will be used in the game. A good DM talks about problem areas with the players and considers reasonable requests by them. The players should realize, however, that the final decision is the DM's: not theirs, and not this booklet's! If a disagreement holds up play, the DM may make a temporary decision and talk it over with the players when the adventure is over. If a player disagrees enough, he or she may quit the game. It is up to the DM to create an adventure the players can enjoy."
In a nutshell, a good DM is open to discussing issues with the group, but in the end, the DM's decision is final. If that's not good enough for the dissenting player, "he or she may quit the game."
That last (bolded by me) quote is worded vaguely enough that it can be interpreted two different ways: a warning to the player ( if you don't like it, quit), or a warning to the DM (if you don't keep an open mind, the player might quit on you). Judging by the subtle humor evident in much of Moldvay's writing, I suspect this is intentional. Being the "Boss" means more than getting your way all time, it also means accepting responsibility for the consequences of being too pushy about it.
Be that as it may, the two things I like best about this section are "If a disagreement holds up play, the DM may make a temporary decision and talk it over with the players when the adventure is over", and "The players should realize, however, that the final decision is the DM's: not theirs, and not this booklet's!".
Running a fun, engaging scenario is, absolutely, the DM's first priority. If some quibble over a rule or something is threatening to derail the action and use up valuable gaming time (something we can all appreciate as we age and have less and less time to actually game), the DM must be able to make a ruling on the spot and move on, whether the disputing player agrees with it or not - there are other people in the room who deserve to enjoy the session they showed up for, not get sidebarred into a rules discussion for the rest of the night. If, after the session is over, the dissenting player still wants to have his opinion heard, the DM should be open to this discussion, and invite any players who feel like sticking around for it to put in their .02 as well. DMs and players alike will find this sort of discussion is often easier once tempers or discomfort has cooled anyway, and the issue might turn out by end of session not have been such a big deal in retrospect.
If, however, after this frank discussion the DM and player still can't come to an agreement, the DM needs to either be firm on his ruling and able to accept that this may be the end of the road for that player in his or her group, or decide whether to give in and let it slide because the player is too valuable to lose.
Secondly, I like the emphasis on the DM's word being final over "this booklet". This is another crucial point as well. If a DM decides elves don't get infravision or sleep spells don't work underwater, for whatever reason, that's the DM's call, completely, and it doesn't matter what the book says. It's the DM's game, not the booklets game. The players deserve to be heard, the rules do not - they are there to help serve the DMs needs for running a good game; guidelines, not laws.