These sign-up are going to be used as a guideline to determine the best time to run Karazhan.

 While the sign-up is open to anyone, there are some restrictions to the builds I'm going to take.  If you need to respec, I recommend you wait till you're 70 and keyed to Karazhan.  There are far better leveling builds than using a raiding build.

Warriors: Prot spec only.
Paladins: Prot spec if you want to tank, Holy spec if you want to heal.  No Retribution spec.
Shaman: Whatever maximizes your DPS or healing.  Also, minimize your burst damage for the sake of the tanks.
Hunters: Marksman spec.  Your build should maximize AP not crit.  Also, make use of the INT->AP talent as you'll be favoring INT gear over AGI gear in some cases.
Druids: Resto.  You're definiately a power healing class.
Rogues: Maximize your DPS.  Also, we don't need a 50% crit rate so keep that under control.
Priest: Holy/Disc spec to heal, Shadow to DPS.  Where Paladins do spam small heals and Druids do few giant heals, you're going to fill the gap inbetween.  Also the Disc tree will help maximize your mana pool for long fights.
Mage: Whatever you feel comfortable using.
Locks: Something that doesn't rely on your Felguard.  He won't be used.

Remember, Tuesday morning is maintence and Karazhan resets.  If you want a two-night run it'll have to be Tuesday/Wednesday.

Note: You can only run one Karazhan raid group per toon per week.  Once you beat a boss on a toon it will be lock to that version of the instance.