Warframes have different base stats as well. Mag has base health of 75, Power 100, and 100 Shields...while a frame like Loki is 75 Health, 150 Power, and 75 Shields.
Mag is good for damage and crowd control, so it would be your best bet to equip mods that focus on buffing her powers, like power duration, power range and power efficiency, etc. It's also generally cheaper to fuse duplicate mods than it is to use fusion cores.
Shield Polarize is a buff for your squadmates, if your cast misses, there's usually a prompt on the UI saying "target invalid" or something like that.
You can see your squadmates' health/shields by targeting them. They glow red as an indicator that they're carrying an artifact in raid missions.
Missions don't scale to player count. Ideally, you'd be rolling with a full squad.
Drops are shared, but you still have to pick them up.
Leveling up your equipment = more mod slots. From personal experience when update 7 rolled out, I grinded defense missions on Helene (Saturn, Grineer) and Callisto (Jupiter, Infestation) till I could get some good mods into my possession. According to the wiki, each faction has a higher chance to drop certain mods. Same goes for the different types of missions.
@chavtheworld: There are several videos floating around (like the one Rasmoss posted) where the cities are isolated, barely self-sufficient, and hit a 200k population without commercial or industrial zoning. It's nuts.
I've had server trouble and lost a couple cities. Like, "stuck at an infinite loading screen, can't see it on the region map, but people can visit it" kind of thing. There's a 150+ page thread of that same thing (last I checked) in the support forums for this game over at EA. I've enjoyed my time with it when things work, but I don't know man. Things are gradually getting there with fixes, but it's up to you. I'd wait it out for now.