Some things are just basic organization: if you're unpacking a multiple-part campaign from a .zip file, you might want some indication of which part you're supposed to play first.
Other times, there are things arbitrarily missing that are there, presumably, to make the game more difficult. But, suppose you start a mission, and there's no comsat available for terran? A comsat isn't just a way of scanning for invisible units--it is, far more importantly, a way to get a sense of where everything is that you need to do. Lacking a comsat in a UMS map can be like playing in the dark. If there isn't a really good reason for it to be missing, it shouldn't be.
Another thing makers might do to make maps more "difficult" or "veteran" is to have enemy bases basically right on top of the player's natural starting base. Maybe this does make it harder, but it usually also makes no sense whatsoever, and really can take you right out of immersion.
But the worst by far is the simple lack of expansions, particularly undefended expansions. Computers steal resources like crazy, even if they have infinite. But more importantly, the option to early expand vs. the computer can be one of the most interesting parts of any mission. So without expansions, the map is "harder" in some ways, yes--but it's also more boring, and since you'll probably end up just having to camp, it forces you to actually play in a more conservative, less aggressive manner. Which, in turn, can actually make the map easier.
Then there's maps where you're supposed to "discover" supporting bases. This is fine--but if the map doesn't really "start" until you discover one of these bases, it can often seem like a waste of time.
Or consider this minor point:
- Bring Maxwell and 3 Ghosts to the Ion Cannon.
Seems intuitive enough, but then you get there and what happens? Maxwell and 3 ghosts are standing next to the Ion Cannon, but victory isn't triggered. If you're patient (which if you've beaten this mission you must have been), then it's no difficult to figure out what you must do: you must bring Maxwell and each ghost to one beacon each surrounding the Ion Cannon. It's not a "big deal" but compare to this:
- Bring a medic to each of the
beacons surrounding the Overmind.
This, from Starcraft: Brood War, can't really be misinterpreted. It indicates where the units must go. Further, there's something just fundamentally more comprehensible about surrounding an Overmind to tranquilize it, than surrounding an Ion Cannon that presumably has one door.
Further, this sort of little thing actually requires a fair amount of work on the author's part: he has to trigger it so that no matter which beacon Maxwell goes to, the trigger still fires. That means he needed to write 4x as many triggers to achieve a counter-intuitive game design element.