Combat mechanics (as of now):
No shield mitigation. Instead, the ship's entire shield strength regenerates in 20 seconds (which is how long ships take to fire their guns, as well). Firepower got boosted hard, so once you focus two ships' fire on one, it'll start taking serious hull damage. This is partly because battlefleet gothic the tabletop game is all about maneuver, so we really hope we can put in minimum speed, or otherwise this system will be more about gathering overwhelming force than about outmaneuvering your opponent. Well, that and being willing/able to get in close enough to fire both sides' guns, although longer ranged fleets may bring back maneuver to try and avoid that.
EDIT:
For now, you'll generally need two ships of a given class to kill a single ship of that class, and they will do so in 80 seconds. It'll take a while, but not all that long. If you make it into three ships pounding on one, then the (poor poor pitiful) one ship will go down in 40 seconds, which is a pretty big decrease. Basically, the shields will take away the first ship's fire, so once you can focus fire on a ship it's going down. Also, keep in mind that I'm using the 20 second recharge for the first shot as well, so in most situations, two ships will be able to kill a ship of the same class in about a minute. However, keep in mind that this is all based off Imperial Navy firepower and shields, so this may vary. For example, chaos gets more guns that can fire to the side in exchange for the Imperials' prow torps (which aren't included in the previous figures, and are pretty nasty as they can punch through shields), so they can finish a ship off more quickly, while tau get a lot of fighters and can fire all their guns prow, so they can kill ships very quickly, although they suffer against larger numbers of ships where their lower overall firepower starts to be a bigger disadvantage.
./EDIT
And the troops business (at least my understanding):
Ships with lances or bombardment cannon can bombard planets, but it isn't all that effective. The real power in attacking planets is troop transports, which are basically space tin cans stuffed full of troops (except for the exceptions, but they won't be in the first group). These are filled with troops from your planets, so even though they're relatively cheap, they need protection, and throwing them away compromises your own planetary defenses. So instead of pounding the planet into dust, you send in troops to retake it, and transports' cargo is valuable enough to make spamming them a bad idea while leaving them cheap enough to use in large numbers, just not waste.
Sorry I was so verbose, if I got anything wrong correct me, if you get any ideas, please suggest them.