Well to start it off, this all depends on what you truly expect your raiders to do. If you have guidelines about raids and expectations, which I hope you do, then they should obviously know them and respect that. If you have a rule about missing a raid, especially if they're someone the group tends to rely on a little more often, they should follow that rule no matter what. If you rotated some people out because they missed a raid or two, then they need to realize that you already rotated them out and they have to work to get that spot back. A lot of raiding guilds have a definitive rule that if you are expected to attend the raid, minus any unforseen circumstances, then you had better be at the raid. If you aren't or are even late, you can't expect to have that spot when you return.
If you have a lot more than 10 people and just plan on focusing on 10-man instances at the moment, then see if you can form 2 raid groups for the instance. That of course allows people to be in the group without having too much of a fuss. On that note, if someone or multiple someones aren't doing too well with a boss, they may be better with another boss, or just need to learn the strategy more, or need more practice. The gear comes with kills, so if they aren't killing the boss, they need more practice. Once that happens and the gear starts to come in, they'll be able to handle bosses a little easier in the sense of taking damage. But overall, they need to respect you as the GM and need to respect the rules you have laid down, and if they don't, then too bad.