Okay, I've finished playing the campaign and I'm posting my initial thoughts on it (better late than never, right?):
A damn good start I'd say...
The briefing for this mission is perfect IMO. The Overmind introductory speech is short and fitting; you obviously wanted to word it appropriately and it feels as authentic as any official Blizzard material. The added effects are nicely done. I am convinced you got this one just right.
The custom music you chose was rather unexpected for a Zerg campaign, since it sounds so... I don't know, "deep"? Anyway I liked it a lot, it was a good surprise in that sense.
Terrain looks nice. I'd add a few more details here and there, but then again I'm a doodad freak, and it does look more barren and desolate the way it is, which I think is a good way of portraying your arrival on an unknown and possibly hostile world.
I liked the fact there's plenty of expansions to take, even if they're not necessary. Makes the mission more fun IMO.
Difficulty is just right. I remember the first version was really easy, with the Terran just waiting to be killed and not bothering you at all. This time I was worried and saving constantly, which was thrilling. Good job on this aspect as well.
Another plus is that you show a bit of the Terran's point of view in this "first contact" between the two species. There was already some of this in the vanilla DS (if I'm not mistaken), but here you added a little more and it compliments the atmosphere.
One thing I'd suggest is that you insert the "pause game" action for the Cerebrate line at the end, so I can listen/read that bit more comfortably. Also, it says "I'm calling your troops back to the hive cluster" -- is "troops" a suitable word for Zerg forces? I like "minions" and "underlings"; not sure if troops is appropriate.
But that's a minor complaint anyway. While I'm on the subject of nitpicks, I'd say purple is a horrible color pick for Zerg.
Purple and blue are for Protoss or Terran IMO, but that's personal taste.
Awesomeness. Not trying to sound like a fanboy but you did pratically everything right for this mission. I have no significant complaints about it.
PS: I'll edit this post as I finish reviewing the missions. More to come later...