In addition to the "official" How to Build Good Missions from Cryptic (which I'm not sure has been brought forward yet), a number of us in the closed beta played and reviewed each other's missions. Here's some general tips taken from posts, I've arranged them with the most critical at the top:
First, its critical your player know where to go and what to do. Putting the sector and system name in the project description is critical. Hard to emphasize this one enough. If they can't find where to go, no matter how clever your mission is - it'll get dropped.
Players may not always think to use the tricorder or scanners to find something (or it may not work). Providing multiple breadcrumbs (beacons in space, lights on a road, etc.) for them to follow is paramount to them enjoying the mission.
Keep in mind that the player may not play your mission as soon as they get it. The mission description must have enough details so they can get there and know what to do.
The kick-off needs some "lure" to grab the players interest.
The length of dialog is a balancing act. To much and people skim it missing things. To little and its too frustrating as it either breaks immersion (why am I doing this or what am I suppose to be doing)?
Having a great ending is pointless if the rest of the mission is mundane. A slow boring windup is more likely to be a dropped mission no matter how good the ending is. Pay attention to all the little details.
Spelling and grammar are critical in dialog. People drop missions fast if they think they are of poor quality. Take the time to copy/paste to something like Word to correct errors.
Pay attention to chapter titiles and labels. Make sure you give clues for what the player can do to succeed.
If the player needs to find an NPC, give a good description for where they have to go. players get frustrated and are more likely to drop a mission if they ca'tt find an NPC within a short time frame.
Don use all caps! It just feels insulting to the player.
Non-federation or un-klingon like behavior needs an explanation fast for why its occurring, otherwise the player feels cheated.
Players envision their bridge officers in certain ways. When you have them say things or give their opinions you run the risk of breaking immersion for that player. Doesn't mean not to do it, but it is something to watch out for.
Here are some actual quotes:
When I get the mission description, it said to "go back to Pi Canis". But I was still in Pi Canis so it didn't make sense. In general, you should probably stay away from text like that as it breaks immersion.
The mission search profile (project description) says to go to Hatoria, but of course it's H'atoria.
All Caps, ick.
I'm forcing him to go against his beliefs? That seems un-fedlike.
So, why were the undine on such an unimportant place as we were told? That was an awful lot of undine for them to not have some specific reason for being there.
What's with the uppity BO mouthing off on my bridge! Escort him to the bridge for punishment! I'll have order!
Now where the heck is the third station? Hmmm, not at the nav beacon. Well, they were called orbital, it must be behind the planet. Nope, not there. oh oh, this is looking like another wander fest - hate those.
Loved the "Condition Red, all hands to battle stations." Except, isn't that a Fed line?
You spelled Qapla' Qa'pla.
Starfleet captain, during a time of war, looking for a cadet cheating.... hmmm....
Ground flashes leading off to the northwest! Excellent breadcrumbs!
Beam down to ESD? Electrostatic Device? What the? Oh, Earth Space Dock.
Okay, says the console is to the left of the turbolift to the admirals office. Hmmm...There is no console to the left of the turbolift ....