There are 12 boss encounters and the breakdown is such:
5 Boss encounters will require off tanks (2 optional)The challenge factor heavily increases as you move through the dungeon. The first few fights are relatively non-dynamic and have been labeled as gear checks. Progressing through the dungeon the boss encounters require more awareness from your players and can pose problems for lesser groups.
The dungeon is very, very large and has many optional paths. In fact, the majority of the bosses are optional so if a guild were so inclined they could run through a handful of bosses to reach the final boss of the dungeon rather then killing each individual boss. This can make gearing up guilds a little quicker since you can skip the bosses that do not drop loot for the classes in the raid.
Currently we know that at least two pieces of tier 4 comes out of Karazhan. However we also know that 8 pieces of loot are in the tier 4 set (the models are in the game and you have seen the videos of characters with tier 4 boots and belts on). However there are no set bonuses for the other 3 missing pieces in the game. Which leads us to believe that the belt, boots and bracers have not yet been added into the drop tables of the game. It is a possible assumption that the three harder optional bosses, Nethersprite, Shade of Aran and Illhoof may drop these three pieces of loot when they are added.
The final encounter is a boss called Nightbane (undead dragon) that can only be summoned after defeating Prince Malchezaar. Nightbane is a vastly harder challenge then any of the bosses in the dungeon and though there may be groups that defeated this encounter in beta, I have yet to see a video of it. A piece of Tier 5 perhaps (like Ragnaros)? Who knows what we do know is he has multiple phases, hits like a truck and in addition to being a skill check, he is also a massive gear check. The dynamic of the fight is a blend of Onyxia and Nefarian with AoE fears and undead constructs.
To prepare your guild for the coming onslaught of Karazhan I have added some links below. These strategies all work but over time refinements will most likely be made as shamans and paladins frequent each raid group and gear becomes easier to obtain.
Below are links to AmpWoW are boss strategy section where there is a considerable amount of info on each boss not strategy, skills and dynamics.