To clear out all confusion about how the Ignite talent works in 2.0, I decided to make this page. It also mentions how it used to work, so that you can see the difference.

Sections: Introduction, 1.12- Ignites, 2.0+ Ignites, summary, credits

Introduction
First, let's have a look at the talent's description.
Your critical strikes from Fire damage spells cause the target to burn for an additional 40% of your spell's damage for 4 sec.
Simple enough, eh? Seeing as a spell crit is normally 150%, and 40% of 150 is 60, the crit + the ignite should then do 210% damage.
This damage is divided across two "ticks", each causing half of that 40%.
You can find out the tick damage by multiplying the spellcrit by 0.2.

In and before 1.12
It's always been like this. The interesting part is, what happens if you crit again before the first Ignite is over? Before 2.0, it used a system which allowed multiple (and lucky single ones) fire mages to blatantly create damage. 1.12 and before:
* The Ignite debuff was shared by all mages (I'm not sure just how it determined who to credit the damage to, it's not relevant anyway)
* Critting before the Ignite was over would reset the duration and add tick value to the old Ignite, stacking up to five times

What does "add tick value" mean, then? Let's use our old Fireball as the example. The tick value would be 300. Critting again for 1500 would add 300 to the tick value, so it could look like this:

1500 Crit, 300 Tick, 1500 Crit, 600 Tick, 600 Tick

Now, if you do the maths, 1500+1500=3000. However, 300+600+600=1500. 40% of 3000 would be 1200, which is the damage the ignites would have done if the Ignite died between the crits:

1500 Crit, 300 Tick, 300 Tick, 1500 Crit, 300 Tick, 300 Tick

300+300+300+300=1200. You see now, how "chain critting" with fire spells allowed you to create damage? The Ignites simply caused more than 40% extra damage. The bonus only grew the bigger the stack got:

1500 Crit, 300 Tick, 1500 Crit, 600 Tick, 1500 Crit, 900 Tick, 1500 Crit, 1200 Tick, 1500 Crit, 1500 Tick, 1500 Tick

1500+1500+1200+900+600+300=6000. 1500*5=7500. This is 80% bonus damage now! Since it cannot stack further than five (which, depending on gear, number of fire mages, luck and ability to cooperate, could be very hard or relatively easy to reach), it cannot get more "out of hand", than this, you'd think. However, even though new crits would not add tick value, they would reset the duration, which really kept Scorch in the game as you could cast two Scorches in the same time as one Fireball, and with Incinerate you would also get extra crit on it. So with rather measly Scorch crits, you could keep this 750DPS DoT alive. These crits could come from any fire mage in the raid, not just the "owner" of the Ignite. You just needed to crit every 3.5sec or so.

Here is a good example of how crazy it could get, though it's far from the best one. As you can see, I got aggro from it as well. Wonder why, eh. ;)

In 2.0
Now, how does it work in 2.0? Well, it doesn't create damage anymore. A summary would be:
* Each mage has his own Ignite debuff and mages cannot interact with other mages' Ignites in any way whatsoever.
* Critting before the Ignite is over will split the yet uncaused Ignite damage over the ticks of the new Ignite. It no longer has stacks.

*** NOTE: For some mages it still acts exactly as it did in 1.12. I don't know why, but if you have knowledge or theories, please share! ***

So, an example. Let's use our good old 1500 Fireball crit (get better gear, srsly!):

1500 Crit, 300 Tick, 1500 Crit, 450 Tick, 450 Tick

300+450+450=1200. 1200 is 40% of 3000. How did it happen? The 1500 crit generated 600 Ignite damage, however one tick came before the next crit. So there was 300 damage yet uncaused. This was split across the two 300 ticks of the next crit, hence 300+150=450 each.

Here's a real example.

Explanation to that screenshot as follows:

2361 Crit, 472 Tick, 2528 Crit, 742 Tick, 742 Tick
(2361*0.2=472.2) (2528*0.2+(472.2/2)=741.7)

472+742+742=1956
2361+2528=4889
4889*0.4=1955.6

Another cause of worry is, what happens if you crit again before it even ticks? For reference, in the old system this still increased the tick value in the same way, you just didn't get ticks between. You could pretty much say the same for this, though the reasoning is different the result is the same:

1500 Crit, 1000 Crit (Fire Blast :P), 500 Tick, 500 Tick

I will provide you with another real example.

Explanation to that screenshot as follows:

1368 Crit, 1038 Crit, 482 Tick, 481 Tick

482+481=963
1368+1038=2406
2406*0.4=962.4 (bless generous rounding for that extra 0.6 damage)

There is one bug, however, which occurs if you score two crits at the very same time. This will usually only happen if you use a bolt spell (Fireball/Pyroblast) and then an instant (usually Fire Blast) and they land at the same moment. I'm not going to promise it can't happen in other situations but the basic idea is two fire spells critting at the very same time. If this happens, you will only get the Ignite damage from the last spell. Example!

Explanation to that screenshot as follows:

2477 Crit, 1236 Crit, 247 Tick, 248 Tick

I guess everyone can see that it's too little damage so I won't be that elaborate, I will simply prove that it's the Fire Blast's exact Ignite I'm getting.
247+248=495
1236*0.4=494.4 (woohoo, another 0.6 bonus damage)
Thanks to Flopi and Hzlr from #eu-mage for pointing this out!

Another drawback is that if your crit is partially absorbed, only the non-absorbed damage will be used for Ignite damage calculations. Say you crit for 1500, but 1000 is absorbed, effectively causing 500 damage to your target. The Ignite will then be two ticks of 100 (500*0.2=100), whereas if the absorbed damage was included as well, it would have been two ticks of 300. A 400 damage loss.

Summary
Though I of course have an opinion on the matter I will leave it out and only give you a summary of the pros and cons of Ignite.

Pros:
* 210% fire mage crits, can be compared to the 200% of frost mage, destruction warlock, moonkin druid or elemental shaman. A slight advantage.
* It's cool to set people on fire. :D (ok, that was an opinon :P)

Cons:
* The 40% (or 60% however you wanna look at it) extra damage comes from a DoT, which means you have to wait at least four seconds until it's dealt.
* Ignite can be dispelled.
* Two fire crits at the same time will cause only the last one to produce Ignite damage.
* Given a strong enough existent Ignite, or a weak enough crit (think Molten Armor) and the manner in which yet uncaused Ignite damage is split across the two ticks of a new crit, you can actually lose a battle due to that extra crit which delayed your Ignite's damage. (<3 Nathaira)
* Absorbed crit damage does not generate Ignite damage.

You may want to ask yourself if you find that up to 210% crit damage rather than the guaranteed (ignoring resilience) 200% granted by frost or other classes' nuking trees is worth all these drawbacks.

Credits
Written by Cinondra, Laughing Skull PvP (EU)
Enquillion on QuakeNet, or just troll the European Mage Forums and you'll reach me :D

Thanks to numerous members of the European Mage Community for promoting and contributing to this site.

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