The first five missions could have been merged into a single one: Building your base, searching for the "discoveries", and eliminating the enemy. The way you spread them out felt like unnecessary padding, and very repetitive.
As stated before, the resources placement was irritating. When I got to the "gather 10,000 minerals" mission, with all the spread out minerals, I just lost all patience.
As for the campaign's difficulty, it isn't challenging at all; fighting an army of hero firebats or breaking through 3 layers of densely packed missile turrets is only tedious. Terrain design can make a big difference in a mission's difficulty, along with fewer but more clever enemy unit/building placement.
You claimed the missions were more than just "destroy the enemy", but it certainly felt that way, only we had more specific objects to destroy rather than the whole base (
e.g. destroy siege tank). Or we had to destroy a base even if it wasn't directly the objective (
e.g. find and build on the island).
The
"River Outpost" mission could have been a more interesting one in that regard, for example. It begins with the Zerg attacking our outlying structures, and gives a sense of urgency to leave for the island. However they never chase the player beyond their starting location, and we're left with tediously clearing the island full of turrets and bunkers.
What could have been done is pressure the player by having the Zerg harass them with waves of defensible attacks. The island doesn't need to be heavy guarded either; it should look like an attractive location to move to, with a fair amount of resources on it, not just two vespene geysers. Instead you could have Zerg air units guarding/patrolling around the island to incite the player to stay and build up forces instead of lifting off straight away to escape the Zerg. Also make use of the "Fog of War" layer in the editor to show players where they need to go.
I'm not saying "destroy the enemy base" types of missions need to be completely avoided, though. Just to make that clear.
Judging by the overall quality, I assume it's your first attempt at a campaign? I'm glad people are still picking up StarCraft and have the interest to make new maps, so I hope you don't give up after this—It's probably not much worse than my own first attempts.
Pr0nogo makes good points in his video response, it will be useful if you listen and remember what he says, if you haven't already.
I will check out your Zerg and Protoss episodes some time later to see if there's improvement as you say.