This is a list of some of the more common mistakes that are made by novice mission designers (and occasionally by the experts). Tips are included on what you can do to avoid them.
Completed missions should never ship with obvious spelling errors in them. If English isn't your first language ask for some help. There are plenty of people who would be willing to help with corrections..
The way the background editor works can be difficult for some people to grasp which means that you'll quite often see missions with only a few randomly placed nebulae in. Backgrounds add a lot of feeling to a mission and as much care should be taken over them as is taken about getting the right tone in the briefings and messages. One very common mistake is to see mis-rotated planets where the sun is shining from the left but yet it is the right side of the planet that is illuminated. Be warned that there are people in the community who are observant enough to notice this even when flying at 5% hull with a wing of raiders chasing them.
Ever seen Star Trek II : Wrath of Khan? Ever got the feeling that the captains of all the ships in a mission haven't? They're all arranged on the XZ plane. No ships diving in from above or nipping in from below. While it's believable that a small convoy of ships might arrange themselves this way so as to spread their flak above and below them effectively it's harder to believe that enemy ships jumping in would somehow know what plane they were on and leap in on it.
This is quite a common one. If you forget to chain your Return to Base Objective it will appear at the start of the mission. The engine is clever enough to avoid putting up a directive to kill Alpha wing if they aren't present yet but it doesn't do that for every directive. Unless you sure you don't need to you'll have to chain the directive to the event which triggers it. More info is given in the troubleshooting section.
A close relative of the above error. Quite often a FREDder will put in a mission objective but forget to actually put in the trigger because he means to go back and put it in later. Nothing screams out "I haven't tested this mission" quite as loud as this cause unlike the above one this one even has music to draw the player to the fact that you've screwed up.
All major capships and anything which is mission critical should appear on the escort list. Making a ship appear there is very simple. Just go to the ships editor->misc. Tick the escort ship box and assign a priority. All caps should be given a priority. Don't make the mistake of only giving them to the 5 most important ships in the game. If one of them is blown up the next most important ship should then appear on the list.
This one won't affect people playing the mission but it's just plain common sense. While you might think you'll remember the arcane naming system you've chosen to use you almost certainly won't if something makes you come back to the mission in a years time to fix a bug. In addition to making your life easier it makes life easier for play testers as they can give you the name of a faulty event/message for you to fix. Finally when you look over your mission and see an is-event-true-delay SEXP how on Earth can you tell what other event it refers to if all of them are called "event name"?
Some missions only need to use one Return to Base / Land on the Battlestar Directive. What some FREDders like to do is have an event that gathers up all the possible events that can end the mission and chain the RTB to that. However amongst new FREDders only one RTB is generally more indicative of a poorly thought out mission. Basically the designer thinks "Hmmm, alpha will fight this cruiser, kill the fighters and then escape with the convoy". No thought is given to what happens if something goes wrong. When this happens the mission basically continues forever because no thought was given to this possibility.
A more insidious relative of the mission with only one RTB is the everlasting mission. Unlike the previous one there are RTBs in the mission but the player can do something that causes it to never appear. Unlike the above error this isn't just a newbie mistake as quite often even the professionals have failed to think of something the player might try. It is seen fairly commonly in newbie missions though so I mentioned it here. As a mission designer you must actively think of ways that a player could screw up your mission and prevent them. For instance if an enemy ship must survive the mission you must have a plan for what to do should the player disable it.
A close relative of the only 1 RTB error is the only one debriefing error. Basically the same logic is at work here and the mission only has a debriefing for a successfully completed mission. If anything else happens you'll only see "No debriefing for mission xxxx".
Diaspora has certain naming conventions that should be used in any mission unless you have a damn good reason for changing it. Wings are always named after colours and enemies are named using the BSG phonetic language Although there is more leeway in the names of capships expecting anyone to take the GTD Suck My Balls seriously is frowned upon.
Worse than poorly thought out names though is the appearance of Battlestar 3 as this shows that you've put no thought at all into the names of your ships. All capships must have a unique name.
Even in the most intense of battles there should be a few seconds delay between the death of the last ship in one wave and the next wave arriving. Otherwise it looks like the ships were lurking, waiting for the death of their team mates. If you really have to avoid giving the player a breather for some reason you should still use a delay but increase the threshold for the wave so that the player is still busy with the current wave when the next one arrives.
A mission designer should take care to ensure that the weapons available in the loadout make sense. It's far too easy to end up with Colonial missiles available in a mission where you fly a Cylon ship.
Remember to turn off the player's jump drives. Also turn it off for any wingman the player can give orders to, or the player will be able to tell them to jump out.