Weapons, spawns, and often layout... size, balence, etc.. I will try to find a link to a guide for making mlg maps and the rules behind it.
Indeed. it's what the pros play on. And the MLG community are the people that brought the playlist into matchmaking. Is that all you needed to know or are you looking for the details so you can forge some for yourself?
In my experiance MLG maps put gameplay above all else, and will sacrifice variety or aesthtics to do so, if neccessary. They also use drop-spawning so that all teams can know when a power weapon spawns, and are built to accompany for the 110% movement speed of MLG gametypes. Since AAs other than Sprint are not used in the MLG gametypes, some people will add AAs that they find beneficial to the map, most commonly Evade or Jetpack. Other than that, I really can't think of much that differentiates an MLG map from a very competetive standard map.
in my mind, MLG maps are maps with encounters happening as fast as possible while still maintaining safe spawns.
MLG also tries to stick with non-bullet-hose weapons, and no vehicles. I think all movable objects also need to be set to fixed so people can't push them around to jump onto things they shouldn't, and explosives are bad. That's just off the top of my head, but many of these things you should already do on a normal competitive map. Edit: From what I remember, MLG also focuses on movement in gameplay, so try not to have anything the promotes one team holding up in one place for a while. Every map should do this anyway though.
Keep in mind that much of the combat revolves around precision weapons like the DMR, so any frame rate issues while scoped and lines of sight are typically even more important than a standard map. It's a good idea to keep a DMR in hand while forging, and make sure to constantly walk the map on foot while scoping possible trouble spots before they get out of hand.
Actually, having a bottomless clip AR and running around firing constantly makes it much easier to find framerate problems that would be present in a 4v4 match.
I'm referring to scoping, specifically. AR magnification is a bit too strong, and any non-scoped lag will usually be apparent while you walk around. Really, just changing to a Spartan once in a while is already helpful; many people neglect the human perspective until their map is nearly finished.
Nothing really makes a map MLG. An average competitive map can be an MLG map, they just have different weapon sets and other factors such as these. Gameplay is over everything, whether you have to get rid of aesthetics for it or pieces causing frame rate. There has to be places that give significant map control to one team if they occupy it, or some places of high importance to win the certain objective or slayer match. Weapons are on drop spawn times so teams can keep track of the spawns. For example, the rockets spawn at start, 12:10, 9:10, so on. Weapons should also be placed in neutral positions of the map, make sure both teams have an even chance at them. These power weapons are also highly valued, make sure your power weapon set is carefully chosen. The MLG weapons are precision weapons, ie the sniper, DMR, rockets, needle rifle, plasma pistol, and the magnum. They also have no vehicles. Make sure spawns are placed in good locations. Players don't enjoy being spawn trapped or being spawn killed. The maps layouts are usually arena style. Arena style is naps such as zealot and h3 guardian. If you want examples of good MLG maps, go to the MLG forge forums. Don't just throw random cover down and bridges upon bridges. Make it structured and have it flow well.
^This. Nothing but this. Really, the only thing that makes a map a "MLG Map" is the weapon load outs. Forgers here claim that MLG maps are different because of what they're used to seeing from "MLG forgers", but really, most of the maps from the "mlg forgers" wouldn't work well in MLG anyhow. And on the note of gameplay > aesthetics, it's gameplay+aesthetics > gameplay > aesthetics. Aesthtics should help with player orientation. A solid grey map is bad for MLG.