It’s not just D&D, it’s any pen and paper RPG. As I mentioned in the Warhammer 40k thread, I GM a Rogue Trader (space Privateers and British East India Company!) game, and play in a Deathwatch (Space Marines!) game. Our group is three people. I GM RT for the other two, one of my players GMs DW for me and the other player.
As mentioned upthread by Clockwork Melon, my co-players and I see it as a collaborative story we’re telling. So we try to keep our meta-knowledge out of the game where possible, because metagaming will spoil the story, and the story is the ultimate goal. Some people don’t like doing that. They’re of a mindset where “winning” the game is the ultimate goal. These are the meta-gamer, the minmaxers, the munchkins and roll fudgers. The guy who will dick over his co-players to try and get his character the best of everything. A good GM will rein in “That Guy”. A poor GM will often be steamrollered by “That Guy” because he has read the rules back and forth and can quote chapter and verse to try and baffle everyone else with bullshit.
And yes, most gamers will stay in character and avoid meta-knowledge as much as possible, but as with any group of people you run the risk that one or more of them is going to have an ulterior motive (“winning” the game, or worse foisting their fetishes off on other players. But that’s a whole 'nother rant) that is at odds with everyone else who is playing.
And for that reason, in the game that I run, “Rule of Cool” is the king of actions. If you can give me a good, well-thought-out, cinematic (“Cool”) reason for taking an action, I’m more likely to allow it or fudge things so you have a better chance to succeed. That’s how one of my players managed to derail a whole plot-driving combat by succeeding on an improvised weapon attack with so many degrees of success that it was just too awesome to force him to fail (as it would have done by RAW).
I have discovered, however, the best way to avoid metagaming and plot derailing is to NOT have a fixed plot in mind. Have scenarios that can link together in any number of ways and then let your players have their heads. They can’t derail the choochoo if the choochoo grew a 4wd and went offroad three sessions ago
But that takes a lot of ability to think on the fly, and most importantly knowledge of your players (in case you haven’t realised, it’s fine for GMs to metagame, but that’s because you’ve got to prepare this shit in advance, or the story goes NOWHERE).
With regards to moving and combat, the 40k RPGs break it up into “Narrative Time” and “Combat Time”. Narrative time you’re just describing what you’re doing, you might have the occasional skill check if you do something out of the ordinary (for example just walking and looking around a marketplace, you can say you’re doing it and I’ll describe what you see. However if you want to look for something in particular, or want to check for something that is being hidden from your sight, then I’ll ask for a skill roll and describe what you see based on how well or poorly you do on that). Combat time is the turn-by-turn “I move x metres, I stop, I shoot/hit something with my sword/ run away like a bitch”. I’ve had sessions where in 8-10 hours, we’ve had no combat whatsoever then I’ve had other sessions where in the same amount of time it’s almost wall-to-wall rolling and combat. It all depends on where we are in the story at the time.