In Brink however CPs are not respawns, simply a place where you can go to get ammo or change class. Ideally you would already have players of the types needed to complete the objective so it doesn't limit your team unless your team is attempting to take advantage of a class rush. The strategy is from the losing team's perspective, you're losing to a team because thye're making good use of the troops they have but you could cut them off from a troop type they need for a short period in order to get you back on your feet.
My issue with mines being invisible to everyone but the Op is twofold. First there is the dev mentality that nobody likes being killed by something they had no chance of defending against, which has seen limitations put on grenades and snipers, but an invisible mine falls into that category (and it's often one of the most hated devices in other games for that reason). Personally I believe everything can be defended against, but if they're going to take steps to limit certain "annoyances" it seems odd to leave out mines. Second it makes Ops and Engineers more appealing/neccesary than they already were which I believe, based on comments in threads about the two classes, may lead to an even heavier supply of their classes.
While you and some others may take interest in the medic for the right reasons, and some may take interest in them for their XP value, you are not neccesarily in a majority. The Engineer has the turrets, invisble mines, and mine disarming abilities, the Op has mine sweeping and turret negating abilities, making them counter classes. Neither soldier nor medic are designed as counter classes, and both lose a portion of their support appeal to the "ammo at CP" and "regenerating health" mechanics, thus making part of their main focus less appealing, which in turn makes them less appealing to a larger fraction of gamers.
Again, it isn't an "OH MY GOD NO" concern, but it is something to be aware of.
Alright so with the command posts; I know they aren't spawn points, but the idea is still the same. First off, we need a safe place to change up classes and such, so without a CP in our safe haven, you could get spawn killed while changing up. For CP's in the field, this is a viable risk, but along the same lines as removing anoyances, this in my opinion would be one of them. Secondly, while its an interesting idea, I think the field CP's already provide that. Remeber, your not going to want to leave the front lines and run all the way back to changeup classes as you will run out of time for objectives. Simply put, I think having one CP that can't be taken over is a good thing.
Now with mines. I do agree that they could become big anoyances, but I think we will have to see how much damage they do. We know they won't be OHK, so I think it will come down to damage and knockback ability. But if these are both low, the invisibility may be the one way to balance and make them useful.
With the classes, I understand that Engy's and Op's are counterparts, but my bigger point is that people will be the classes requierd for objectives. So if were on a demolition objective, defense will have Engy's, but attackers will NEED soldiers and maybee have a Op for scouting and support. We don't need more than 1 Op to spot these mines or disable a turret, so I don't think we will have an influx of them. With escort missions, the medic is the only one who can revive the person (at least in person escort missions). So again, I think classes will be based upon the current objectives and not on special abilities. Also, here's hoping turrets are not OP and can not be placed in extrememly frustrating places, like they were in TF2.