Ignis is a relatively consistent fight, but it's one that requires coordination and attentiveness. It's the first of a new breed of fights in Ulduar, which really put a great deal more focus on how fast your healers can switch targets, and how well your tanks can move adds and the boss.
You can think of the Ignis fight as an endurance fight. While no particular portion of it is going to be incredibly harder than another, it will continue its cycle for quite some time. The trick here is to maintain, and stay diligent about blowing up adds.
There are three basic facets to the Ignis fight that you must understand to succeed. First, he will occasionally pick up a raid member and toss that member in his "Hot Pocket." Second, he puts scorches on the ground, through which you must kite his adds. These adds are called Iron Constructs, and are the real focus of the Ignis encounter. Third and last, you have to watch out for Flame Jets.
Periodically during the Ignis encounter, he will grab a member from the raid, and stuff it in his Slag Pot. When you're in the Pot, you can't attack. You can heal yourself, though. While hanging out in the pouch, you take immense damage every second. It's really going to be a test of your healers' abilities to make sure you live through it. If you do live through it, however, you are rewarded with 100% haste for about 10 seconds. You want to be sure to capitalize on that time, and do big DPS immediately.
Every so often, Ignis will cast an ability called Flame Jets. It has a short cast time, but it's enough warning to cancel all of your casting. Why would you want to do that? Because if you're casting a spell when Flame Jets completes, you will be Interrupted, and unable to cast spells for a few seconds. Especially for healers, that short time of interruption could spell a wipe for your raid.
Scorch and Iron Constructs
While the Hot Pocket and Flame Jets are certainly interesting dynamics, those aren't really what Ignis is known for. His real trick is Scorch, and how it affects his Iron Constructs.
First, you should know that Ignis activates the rows of statues along each side of his courtyard, which will animate and be called the "Iron Constructs." While these adds aren't very frightening on their own, Ignis receives a damage buff for each construct active. We'll talk about how to kill them in a moment.
Ignis will periodically cast Scorch in front of him, about dead center where the tank is standing. The ground affected by Scorch will become a burning zone of fire. In the grand tradition of fires in every other boss fight, do not stand in this fire unless you have a tactical reason to do so.
This is how you kill the Iron Constructs. You must drag them and hold them in place in the middle of an Ignis scorch patch. While standing there, the Iron Construct will gain the debuff Scorch. Once they've reached a stack of ten, the Iron Construct will become Molten. As soon as it is Molten, it will wipe aggro. Move the Construct to one of the pools of water on each side of the courtyard. It will become Brittle. Once the Iron Construct is Brittle, a single large crit will destroy it. No amount of small damage will do it --- only a single large hit can. You need a single hit of 5,000 to get the job done. When the Iron Construct explodes, it will deal a huge amount of damage to the area around it -- so keep your distance.
Putting it all together
So, this is how it works in total.
Your main tank will pull Ignis. Ignis will activate an Iron Construct. An offtank will grab the Construct, and drag it into the Scorch patch. You have two options to keep the Construct there -- root the Construct in place to allow your off-tank to get out of the patch, or have the off-tank stand in the patch and take damage. (You can, in theory, hover on the edge of the patch to allow the Construct to get the debuff, without the tank taking damage -- but latency can make that unlikely.)
Once the Construct's molten, get it into the water. The off-tank can drag it there, you can let residual healer aggro lead the Construct over, or maybe have a Death Knight grip it into the water. Then, you're On-Demand Big Critter hits it with a big shot, and the Construct dies.
Healers heal the tanks and raid, while being careful to take care of the person in the Slag Pot, and taking care not to get locked out of their spells by the Flame Jets.
The exacts of positioning this fight depends on personal preference. I prefer to tank his Scorches in a pattern such as in the picture below, so that I turn him back and forth like a dial. I start the first Scorch where I marked "1," and rotate him back and forth.