1. no mid-mission saves - eek. Given that some missions can take close to an hour, I don't like this. Even quitting out of a mission advances the world by a day.
I completed the
DoW 2 campaign at "Sergeant" difficulty, and the average mission length was less than ten minutes. I don't know how things change at "Captain" or "Primarch" levels, but the maps are fairly small and have direct objectives -- what is taking up to an hour to complete?
2. no idea how to prep for individual missions. Am I missing something? How do you know which teams to bring along? how to gear for them, etc?
As you capture Communication Arrays, you'll start getting pre-engagement intelligence about what to expect on the ground. "Target is vulnerable to plasma weapons" or "Target is protected by swarms of infantry," that sort of thing.
This campaign is going to have a high level of replayability, and I think it might be a good thing if we can share tactics, or maybe have AAR's or something that would give someone an idea of what to expect. Not necessarily plot spoilers, but something like : 'have avitus bring the missile launcher for anti-vehicle'
Generally speaking, I used my Force Commander to charge in and knock ranged attackers out from their cover, Avitus to lay down suppressive fire, Cyrus for anti-building demolitions and HVT sniping, and Tarkus for a little of everything. There are two other playable characters you'll unlock soon enough, but I suspect you'll be comfortable enough with the system to know how to use them.

I also liked keeping my Force Commander loaded up with healing items like Stimulant Packs and the Chapter Banner. He's tough to kill and frequently in the middle of the fray, so it's easy to have him toss off a heal to wounded squad to anyone in range.
-Autistic Angel