Grobbulus is an unpopular fight on the tanking forums at the moment due to what are assumed to be new mob pathing mechanics implemented in Wrath. No one's exactly clear on what Blizzard's changed, but the popular theory is that servers are attempting to compensate for player lag by estimating where you're likely to be based on your movement, rather than where you actually are. This has led to a lot of mobs pathing straight through moving players while aggroing, which is something of a nuisance for tanks attempting to position and generate threat (particularly bears, as Druids do not have 360-degree threat generation).
Otherwise, Grobbulus is significantly easier than Patchwerk. He's an execution fight, and isn't very punishing on an undergeared raid as long as people are careful about watching their debuffs.
- Poison Cloud: You'll see him start to spawn this under his feet every few seconds (watch for it; the graphic is pretty obvious). At first the cloud isn't too big, but will grow to more than double its size, so your tank will need to back up a sufficient distance not to be caught in it as the cloud does about 2-3K a tick nature damage on Naxx-25.
- Mutating Injection: A random player is "injected" with this disease and, after 10 seconds or upon being cleansed, will explode for roughly 10K nature damage to themselves and any player within range and leave a Poison Cloud beneath them. Ideally you want to make sure each player is safely on the side of the room before the disease is cleansed or they explode; otherwise the Poison Clouds are going to be left in fairly inconvenient or dangerous areas for the raid. Injected players will need to be healed once they explode. As the fight goes on and it may become more difficult to find an area alongside the wall not covered with Poison Clouds, they may also need to be healed through the damage they'll take from preexisting Clouds while they're waiting to explode. As Grobb's health drops, he will do this ability more and more often, to the point of chaining it constantly as he drops below 10% health.
- Slime Spray and Fallout Slime: A frontal cone AoE nature damage spray that will spawn one Fallout Slime per player caught in it. If your raid is moving correctly, only one slime -- the tank's -- is going to spawn. Slimes are tauntable and should be picked up by an offtank.
DPS: Typically my own guild has ranged DPS on Grobbulus and keeps melee on the slimes. It's entirely possible to reverse this, or to keep the entire raid on him with the exception of some ranged DPS on the slimes, but it does increase healing load as melee will almost certainly take damage from the spawning Poison Clouds (it also increases the chance that you'll wind up spawning a ton of slimes, as he can chain Mutating Injection and Slime Spray fairly close to each other).
If you tend to run with a very melee-heavy raid and you won't be able to avoid the enrage timer (9 minutes on heroic, 12 minutes on normal) without putting melee on him, your tank will need to kite Grobbulus a little further to ensure that there's an unmarked spot for melee DPS after each Poison Cloud, and melee will need to attack him from maximum range to ensure they are never within his hitbox. Players with Mutating Injection cannot explode or be cleansed anywhere close to the boss with melee on him either. Otherwise, DPS is pretty easy. Keep moving so that you're never in front of Grobbulus as the tank kites him around the room, and watch your debuffs so you're well out of range of the raid when you get Injected.
Healers: This isn't a particularly difficult fight to heal as long as Injected people explode along the wall. Grobbulus' melee is respectable but not scary, especially if your tank already has a few purples. The main danger to the raid is too many Fallout Slimes being spawned if people get careless about where they are in relation to Grobbulus, or a person being cured of Mutating Injection too early. If you typically use mods like Healbot and Decursive, resist the urge to cleanse as soon as you see the debuff pop up. And whatever you do, shammies, do NOT drop Disease Cleansing Totem. An additional warning for Paladins with the Holy talent Pure of Heart: you will have only 5 seconds to get to the wall, rather than 10.
10-MAN vs. 25-MAN
No real difference apart from increased damage on heroic. There's also no Achievement associated with Grobbulus unless you count The Immortal and The Undying.
I hope this helps, and we'll see you folks tomorrow for Gluth and Thaddius!