Your best friend is the campaign. If you haven't beaten it yet, do that before reading the rest.
It provides a lot of good encounters and what to do.
So before we even start, is take a look at the campaign encounters.
Here's some checklist stuff:

• Are the enemies close together or far away from each other?
  
• How many enemies are there? Count heavies and super heavies separate from the fodder.
  
• How close-quarters is the encounter? Are the enemies far away, or are they close?
  
• How many supplies are in the arena? (health/armor/ammo/power ups)
  
• How long was it since you saw a shop? (Gene Machine/Gun Nut/General Store)

All of these are an optional exercise, but here is the main point:
1. They are always close together.
2. There's always plenty a heavy and some fodder.
3. Most encounters are close-quarters, and the only discrepancies are when you have to use the special sniper to teleport.
4. There are always a medium amount of resources, to make the player use more different weapons.
5. A shop usually appears after 2-3 encounters to allow the player to spend their hard earned cash on ammo and augments and weapon upgrades.

Here are some things that you should NEVER do.
- Place enemies far away from the player except for a situation where they have to use the Telefragger. In that case it's fine.
- Spread enemies out.
- Make a maze.
- Make your encounters stupidly long.
- Make your encounters have too many or too little enemies.

Here are some things that in a standard map should probably not go, but it can work if it does.
- Add lots of barrels.
- Make the player start with no weapons and add weapons to their arsenal as they go on. You can use the player inventory logic item in the same tab as the player spawn to control what guns they have.
(not much huh)

Here are the things you ABSOLUTELY WITHOUT EXCEPTION do in a standard level. This is stuff that is not already covered in the Don'ts section.
- Pace out your encounters with parkour segments after big encounters or after shops.
- Make your level visible. By that I mean be able to distinguish enemies from the background.
- Guide the player on where to go. Even the campaign fails at this.
(not much)

An encounter is defined as a string of small encounters, usually with one or two heavies and some fodder. Three of these is defined as an encounter. You can also just have one big arena, but make sure to give the player some ammo and maybe a power up in the arena, because things get hectic.

Not stuff that's necessary for a good map, but good to have on hand.
- Fodder can be treated as supply drops.
- Enviromental hazards can be really good if used correctly.
- Barrels can be used as a hazard and a weapon.

This guide is meant to help increase the quality in maps in the early stages but is very incomplete. I love this game and would love to see other people make very good content and I hope this is a base for those maps. Please note that non-standard maps (speedrun maps, jokes, and other stuff not like the campaign) are free ground to be experimented on. Come up with your own style and whatnot.