@Sagath

Not 33%, 50%. You got the fraction upside-down.

X% haste doesn't "reduce your cast time by X%", it allows you to "cast X% more spells per time period". The formula for cast time is 1/(1+haste), with haste expressed as a decimal (ie, 15%=0.15).

It's probably been done this way to stop people from being able to make everything an instant cast by gearing like crazy toward a 100% haste cap. The model of haste we have in WoW has 100% haste giving you twice as many spells in a unit of time, so it's halving your cast time. It's worth noting that X% haste will cause an X% increase on your unhasted DPS, but also makes you burn through X% more mana per second. That extra mana burning on haste compared to crits (which don't cost any more than the normal spell) coupled with interesting procs on crits is the big reason why 1% haste costs so much less item budget than 1% crit.

I theorycraft for my shaman girlfriend quite a bit, and it's pretty easy to tune her haste around her rotation. Let's say she wants to fit 5 lightning bolts into the 8s cooldown of her lava burst. I take the 10s total cast time of 5 lightning bolts and divide by the 8s of the lava burst to get 1.25 so she needs 25% haste. With her current gear that's just too much to bother trying to enchant/gem for, so instead she's gone for around 18.75% and squeezing 1CL+4LiB (9.5s worth) into the 8s.