So in Cataclysm Haste is going to affect DoTs. This, developers say, is necessary to avoid having Haste be much better for some class than others. And they are completely right about that. Having Haste not affect half the damage of some classes means that it is a wildly swingy stat. Stats will always have some variability in usefulness for different classes, but having them relatively close together is useful. It is how Haste will affect DoTs that is the problem.
Right now Haste can affect a handful of key DoTs in the game through talents or glyphs. Haste simply shortens the total duration of the DoT. So your DoT used to tick 5 times in 15 seconds and you have 25% haste, now it ticks 5 times in 12 seconds. At some point there was a blue post that said that haste was sort of a boon but also in a way a drawback on DoTs because you ended up casting them more. I sort of shrugged that off at the time - after all, I can't get myself upset every time someone says something that is wrong on the internet - but apparently they had a plan to put their wrong-headed thinking into action.
Suppose you have a 15 second DoT (3 second period) and a 1.5 second nuke (I choose these numbers so everything rounds well, obviously). You cast your DoT, then you cast 9 nukes, then you cast your DoT again. Your DoT ticks for 2k and your nuke hits for 5k. So in 15 seconds you do 5 DoT ticks and 9 nuke hits for 55k damage.
Now you get 25% haste. If this shortens the length of the DoT then you cast the DoT every 12 seconds. How many nukes do you cast in the intervening time? Well, they now take 1.2 seconds. With the GCD to cast the DoT you cast you have 10.8 seconds left. 10.8/1.2 = 9. So you cast nine nukes, the same number as you did without haste. So you do your 55k damage in 12 seconds instead of 15. That means you do 55k/12 = 4.58k dps instead of 55k/15 = 3.66k dps. So 25% haste gives you a 4.58/3.66 - 1 = 25% damage boost. Yes, 25% haste, 25% more damage.
They are saying that they want DoTs to tick more often but have the same duration when you get haste. In that case, the 15 second DoT would still last 15 seconds but would tick every 2.4 seconds, getting an extra tick in there. They are also saying that reapply a DoT will extend the duration without resetting the tick timer, which means you probably want to reapply before it falls off1.
So now you cast your DoT and then you'll want to cast it again when the last tick happens before the 15 seconds elapses. So you cast the DoT, wait for the 1.2 second GCD, cast 11 nukes in 13.2 seconds leaving you 14.4 seconds into the DoT duration. You don't have time to cast another nuke so you reapply the DoT. Now you get 6 DoT ticks and 11 nukes in 14.4 seconds. That's 67k/14.4 sec = 4.66k dps. Under this system a 25% haste buff gives a 4.66/3.66 - 1 = 26.8% damage boost.
Of course that is a very simple example. I ran a simulation for shadow priests using my own current average damage numbers for each spell to get the correct ratios between them. With the current system 25% haste gives just over a 25% damage boost while 50% gives just under 50% damage boost over a 5 minute run (difference is accounted for by clipping). With the new system, 25% haste would give a 28% damage boost and 50% haste would give a 54% damage boost.
It's not really that big a difference, but on principle it is crazy. DoT classes long suffered from getting little to no benefit from Haste rating. Now they will get a better benefit than others. Why go with this system instead of with the system that makes the benefit identical? I assume its because they really think that casting DoTs more often puts them in a disadvantage.
Guess what, Arcane mages with more haste cast Arcane Blast more often. That's what haste does.
1. In the example given, choosing not to refresh the DoT would give 6 DoT ticks and 12 nukes every 15.6 second for 72k/15.6 = 4.61k dps, a very slight dps loss over the alternative and refreshing before the DoT falls off. This is because if you refresh before the DoT falls off you get a tick every 2.4 seconds, whereas if you let it fall off you "waste" the last 0.6 seconds of each cast.