Here are some tips and tricks I can offer my dps DK brethren whilst stepping foot into the mighty Firelands. I'll cover the first 5 bosses for now (the first "wing" of FL) and I'll cover the last two bosses in more detail once I get more comfortable with them. As a general rule of thumb the idea is survival. You can't dps when you are dead, right?
Grind rep and do dailies, no fake. I recommend trying to get honor as fast as you can. Look for rep runs forum in trade, ask your guild, start one yourself - it's worth it. You can get a 378 cloak and belt for nothing more than killing mobs which also has the chance to drop BOE 378 epics AND crafting recipes! It really is that easy. The cloak and belt are VERY good for DK's - I plan on using mine for quite some time. Also the Molten Front dailies are a great source of some gear. As you unphase the dailies more and more 365 gear will be available to offer a minor upgrade to a 359 should you need it. Another benefit to unphasing these are access to crafting patterns. If you have a crafting tradeskill these might become useful and/or profitable. There is a "help Thrall" quest line that is unrelated to to MF dailies that you can use to get a 365 cloak right out of the gate. I recommend that as well.
Firelands is a raid where utility is almost as important as pure damage. I've modified my spec to include Chilblains which isn't something I would traditionally do. The added utility has made a huge difference in a variety of encounters. I highly recommend picking this talent up.
Download and learn Power Aura Classic. The easier it is to know what is going on the more attention you can divest in situational awareness and rotation. I have alerts for my Apparatus of Khaz'goroth, my Pillar of Frost cooldown, Horn of Winter buff, a finally a 30% HP warning. The Blizzard UI does a nice job of showing you RIME and KM procs but you could of course replace those with POWA alerts if you see the need to.
There really isn't too many tricks to handling FL trash. If you have taken my advice from above and spec'ed into Chilblains you can effectively lock down a turtle in the turtle packs with Chains of Ice. Considering melee can only get into dps the patriarch, you might as well lock down a small turtle and save some lives. Besides, it's fun. The spinners leading up to Beth'tilac do a crapton of damage and often are neglected in the back. Death grip one and kill it. They are squishy. Do not be afraid to thrown down a D&D on the scorpions and pay waste with Howling Blast. Just be prepared to pop an AMS when they start dying - they have a stacking debuff and you can easily, EASILY die when they do. The large fire naga packs are tricky at best. Make sure you are getting on a non-CC'd caster and interrupt your ass off. If someone pulls and things start to go pear shaped, don't forget about your Hungering Cold. That few seconds of reprieve can be a game changer.
The important thing to remember here is there is no taunting the accompanying dogs. Being the upstanding killing machine you are it doesn't take much to get a little proc-ie and wind up tanking. If that happens prepare to die because these dogs do hit very very hard on plate dps. Just keep a muzzle on it until the last phase. When Shannox throws his spear there will be some AE damage and a fire glyph on the ground (think last boss in the Twilight Highlands CaC). The AE damage is unavoidable (but can be mitigated) but you can avoid the glyph on the ground. Use AMS during this to help the healers. If you are running DBM turn on your speech bubbles so you can see when there is a trap on your feet or someone near you.
You may be driving his legs on this fight. If so, it's all about experience with a dash of luck. Use AMS and IBF early and often. There can be tons of raid damage on this fight so use them liberally. When he sits down BACK OFF. His threat table doesn't turn on until that phase but if you continue to beat on him before the tank picks him up that is more than enough time for you to eat it. Just wait until it's safe then unload on him.
This fight can be SUPER fun for us. My guild actually uses me to kill the spiderlings by myself. Just make sure you have Chilblains and you have glyphed Howling Blast. This will keep the 'lings slowed while you kick their faces in. If you are having trouble kill them glyph your D&D (for a longer duration) but try to avoid that. It makes it harder in the last phase to lay the hurt on Beth. Keep in mind if there is no drone active the 'lings will aggro you and they can kill you faster than you can say OUCH. If there is a drone active don't worry about aggro just bring the pain. Use AMS right away to mitigate some of the fire AOE. You will be running around and might be outside the healers range but you will still take the occasional fireball. Use AMS often. Save your IBF for the later half of phase 2 when the stacks get high and the healers will be strained.
On the ground your job will be to keep the interrupts on the initiates. They cannot be gripped as an interrupt so don't bother. If you are fast you can taunt them right before they shoot their fire patch and cause it to go into a wall or toward an area where there are less raiders. Make sure you get at least one feather per ground phase. No more, no less. For the tornado phase a great tip is to run behind one until another one passes you. Get behind that one and follow it until another one passes you. Rinse repeat - you'll essentially be making a large circle. Save your IBF for phase 4.
This fight is simple. Just make sure you have the rotation set and clear with who is taking which beams. I suggest having the first person hug the crystal and when they need to swap run toward their replacement. They can even run toward the crystal when they see that to make it easier. As a DK you can eat some of the damage with your IBF and AMS. Just use it once you have 3-5 stacks. They healer will thank you. Oh yeah, and dps your ass off.