Haste thresholds are like hit caps but far more complicated. There are multiple haste thresholds that apply under different buff scenarios, and you have to decide with haste threshold you want to aim for. Then, again, every time you get new gear, you have to reforge like crazy to get yourself right back at the correct haste threshold. In practice, I'm hardly bothering. As a result, Ohken has a significant amount of haste right now that I should really get around to reforging off.
It doesn't help that the mechanic doesn't exactly feel polished. There have been a lot of questions posted over where the exact thresholds are. Many people are grabbing 10 or more extra haste just to be on the safe side. It also doesn't mesh well with the warlock spell Dark Intent. If you cast it on someone who is just above a haste threshold, they get minimal benefit from it. However, if they want to reforge so as to take advantage of it, they are severely gimped whenever they don't end up with Dark Intent. Overall, it feels like I'm doing a lot of work around haste thresholds for something that, from Blizzard's point of view, is more of an accident than something they prepared for a gamer to experience.
Let me describe a way haste thresholds could be removed from the game. First, consider the background. Up through most of Wrath, haste didn't help HOTs and DOTs at all, and thus it tended to be weak for classes that rely on HOTs and DOTs. In Cataclysm, all HOTs and DOTs now scale with haste. The way it works is that haste improves the ticking speed but does not change the duration of the spell. If you have a spell that, at baseline, does 5 ticks in 10 seconds, and you get 100% haste, then the spell will do 10 ticks in 10 seconds.
The only problem is that now we have to deal with haste thresholds. If you have anywhere between 80% haste and 99% haste in the above example, the spell will do 9 ticks in 10 seconds. Thus, the haste from 80% to 99% isn't improving your character in the slightest. That's a large gap for a stat to be doing absolutely nothing!
To eliminate these gaps, all it would take is to add a partial tick whenever a HOT or DOT is about to fall off. Continuing the example, if a player has 90% haste, then the spell would do 9 ticks at full effectiveness and then a partial tick at half effectiveness. With 81% haste, there would still be a partial tick at the end, but it would be a tiny one. With 99% haste, the partial tick would be almost as strong as a full tick.
One rule of game design, one I think is widely held, is that you want to control the kinds of decisions players are faced with. You want those choices to be fun. As an example, deciding whether to cast a big heal or a small heal is a fun decision. Each decision takes about 100 ms to work out during a single battle, it's not punishing if you make a mistake on some of these decisions, and in a gaming session you make a long string of these simple decisions.
I think haste thresholds fail the fun test. The only way to even know what the thresholds are is to do extensive theorycrafting and play testing. Hardly any player has time to do this themselves, so what they do is consult sites like Elitist Jerks that have a complicated chart of thresholds and a guide on which ones to aim for under what circumstances. It reminds me of the cookie cutter talent builds that we all currently use. Like with cookie cutter builds, reading up on haste thresholds are something you have to do and then, like a monkey, replicate in the game. There's no real choice in it, and it's tedious going through the exercise.
I think haste thresholds should go, and I think I see a way to do it. WTB partial ticks.