If you haven't read anything on this boss yet, it's the single largest gear check in WoW yet. It's Burning Crusade's Patchwerk, mostly. To beat Brutallus, you need roughly 29,000 sustained DPS across your entire raid. If you don't pull that off, you hit his enrage timer and he destroys all of you. Simple as that! If you're lucky you can burn off a final two or three percent of his health after the enrage, but that's about as far as you go. That three percent is about 300,000 health, so don't get too confident.
Hybrids at their base naturally do less damage than 'pure' damage dealing classes. Understandable. As I've said before, though, their utility can be simply mind-blowing. The lower DPS on the Hybrid classes can easily make raids nervous going into this encounter. Assuming your raid is using two tanks for Brutallus as most strategies suggest and the typical seven healers, you have sixteen damage dealers to bring down 10,500,000 HP in six minutes. If my math is right(and I'm pretty sure it is), your damage dealers will need to do a little over 1,800 DPS each, over the entire encounter.
This may not seem like that much to the pure damage classes that see that number, because it's probably right around what you do in a raid normally. Take some things into consideration, though. A Shadow Priest, which gives the entire raid 5% added damage to spells, in addition to an extra 10% Shadow damage for the Warlocks, will be doing, at most, around 1600 DPS in a properly stacked group. Less than that if they're augmenting the healer group, probably somewhere around 1450. Assuming you only have one Shadow Priest(my raid has two), that's an extra 25 or so DPS per person to put out. If you run two Shadow Priests, one for the healers and one for the ranged DPS, that's an extra 50 per person.
That number seems fairly small. 50 per person? That can be made up. Is it worth it to use Shadow Priests? Very, very yes. Consider their utility. Vampiric Embrace, Vampiric Touch, Misery, Shadow Weaving. These are all very good things. Their contribution begins to diminish after that first Shadow Priest, Misery and Shadow Vulnerability are not a concern after the first. However, Vampiric Touch is still quite amazing. A Shadow Priest in your Healer group strengthens your healers, and lowers the chance of tank deaths. A Shadow Priest in your ranged DPS group means less need to Life Tap, Evocate, things of that sort. Even without the direct damage buff, Shadow Priests will give your damage dealers the longevity they need.
Shadow Priests can't do the suggested 1800 DPS, no. They do push Warlocks far above and beyond that requirement, however. A Warlock doing 1700 DPS without a Shadow Priest is doing just shy of 2000 DPS with one. That's just one warlock. Your raid probably has two or three. This is anywhere from a 600 to 900 DPS increase from just the Warlocks, not to mention the smaller boost to the Mages. That certainly makes up for the slightly lower Shadow Priest damage, doesn't it?
Every raid has, or should have, an Enhancement Shaman. Let's pretend you cap out at 1500 DPS in a raid-buffed, melee-stacked group, in a pure free for all fight in which threat is not an issue, like this one. If your groups are organized properly and you've cleared Black Temple, this most likely is not the case, you can do more than that. For the sake of discussion, though, that's your cap. Enhancement Shaman fall short of the 'required' sustained DPS, but again, not an issue. Totems, Heroism/Bloodlust, Unleashed Rage, all of those sorts of things turn your melee into monsters. Between you and the Arms(or Fury) Warrior, 1600 DPS rogues turn into 2200 DPS balls of destruction.
None of this includes Improved Faerie Fire, Totem of Wrath, or Improved Seal of the Crusader yet. Each of these push your pure classes higher and higher.
As we're going through this list, you really notice that the statement of "everyone must do 1800 DPS" is actually rather misleading. It frightened me when I first read it, too. How the hell am I going to do 1800 DPS!? I included that number early on to to grab your attention, but don't make yourself feel like dead weight if you can't achieve that. The goal of the encounter is not to make all of your damage classes do 1800 DPS. The goal is to utilize your hybrid classes to the best of your ability, to make the most out of their utility, and turn your pure damage classes into swirling monsters of death.
There is no hybrid that is dead weight on this encounter. From Retribution Paladin to Balance Druid to Elemental Shaman to Fury Warrior, their contribution is substantial. This encounter's focus is not personal glory. While you should always strive to better yourself as a player, the focus of the Brutallus fight is raid synergy. The need to 'stack' your raid for one thing or another is minimal. You simply need to work together to overcome the hurdle, more than you've had to in the past. Synergy is key. Who better to help that synergy come to fruition than the Hybrids?
Since the dawn of the Burning Crusade, Blizzard has worked hard to make the best out of the Hybrid classes. They've done a lot towards making each of these classes and specs worthwhile, and this encounter takes that even further. Hybrids are not just viable, they are essential. I don't know about you, but that makes me feel pretty good.
If you want more information on this boss, especially if your raid is approaching this content, the Elitist Jerks forums are an amazing source of information. Remember: Check existing threads before posting your own! There's a ton of informative threads there, and your question has most likely been answered in one of those threads already. If it hasn't, your question probably doesn't need its own thread. It could go in one of the others.