Of the 4 end bosses, Maexxna is probably the one you will see first because the rest of the spider wing is quite easy.  The fight is all about DPS, as a tank, you don’t need to worry about anything until the boss gets to 30%. Periodically throughout the fight the boss will spawn a whole bunch of little spiders that the OT can pick up and the DPS can aoe down pretty easily.  The boss will also randomly cocoon people and throw them against the wall.  The cocoon does some damage to the person affected and you will want to have a few of your ranged DPS assign to shoot down the cocoon to free the people trapped in them.  The boss can use Necrotic Poison on the tank which reduces healing by 90% but it is cleansable so you can have a resto druid keep abolish poison rolling or have a pally cleansing every time it is up.  The boss has an ability called web wrap that will incapacitate the entire raid for a few seconds so the healers will want to keep a full stack of hots on the tank.  Its all pretty straightforward but at 30%, the boss enrages and will start hitting a lot harder and faster.  Web wrap while the boss is enraged can easily kill the tank because the increased damage, the lack of dodge, parry and block and because all your healers are locked down for a bit.  In order to kill the boss, you want to have as few web wraps during the enrage as possible.

When the boss is at around 35%, you should slow down dps and wait for a web wrap to happen so that when you push her over into the enrage, you have as much time as possible before the next web wrap.  Have all your dps pop cooldowns after 30% and burn as hard as possible.  Unless you have just insane dps, you will get at least 1 enraged web wrap so be ready to pop last stand or shield wall in order to survive it.  Unless your dps is really good, you will probably get at least 2 enraged web wraps so don’t use both cooldowns on the same web wrap.  If you have a prot warrior OT, it helps if they intervene the moment web wrap ends so your healers have an extra second to catch up.  If you use last stand on the first web wrap and shield wall on the second and it looks like you are about to have a third, make sure your healers are ready to pop their cooldowns for it.  Between priests with guardian spirit, paladins with bubble + hand of sacrifice and your own cooldowns, you should have no trouble surviving multiple web wraps.  But if someone screws up and you go into an enrage web wrap without a cooldown from anyone, you are probably going to die.  The boss is tauntable though, so even if the MT dies, the OT should be able to jump in and take over.

Ideally, a group that is new to the fight should be able to get through the fight with only two enraged web wraps.  If you see three, then your dps is behind and they need to step it up or manage their cooldowns better or manage the 30% transition better.  If you see more than three enraged web wraps, your dps is having serious issue and would perhaps be better off playing an easier game like Connect Four.