In 5.4 haste is not necessarily going to be your friend. Our stat weights will change. In 5.4 items are being redesigned. Currently, haste is great because it increases your opportunities for procs to occur. And if a proc causes even more haste, then it's even more great. If it causes our main stat to buff, then it's great as well because along with the haste you're doing a rolling and growing snow-ball of damage as your gear improves.
What can proc in the revised Real-Procs-Per-Minute (RPPM) model? Lots of stuff and the same stuff as now, most notably: Trinkets, tier bonuses, and legendary meta gems. When 5.4 lands in two weeks, we'll want to change a few things. One thing we'll really want to do is tend to our revised and redesigned trinkets.
I am using two trinkets now. I have the upgraded form of the Heroic Renataki Soul Charm and the upgraded form of Bad Juju (Gnomes!). Well these are turning into Renatakai Soul Charm (PTR) and Bad Juju (PTR).
You can see one difference if you look closely. The duration of the effect is 20 seconds now for both, which is a nice size duration and makes the soul charm quite good. But the PTR forms are coming in at half that duration; They have 10 second durations. The soul charm still gets up to the same buff. It gets better every second instead of every two seconds, but it will have less output and be more difficult to get a couple large shots in or anything big at its peak.
There are a few things that isn't listed in tooltips. First, the Real Procs Per Minute have a "may the odds be ever in your favor" unlucky-streak balancer. If you haven't gotten a proc in a certain time it buffs and buffs your chances until you're guaranteed to. Overall, this works out to be a 9% bonus to your chance to proc. The second is, that on a boss pull, they're putting you high up in that chance so you'll get a proc early in a boss pull. The third is that most of the chances for procs are nearly doubling. The Soul Charm currently will have on average 0.62 RPPMs which is becoming 1.21. And my Voodoo Gnome Generator is going from 0.55 RPPMs to 1.10 RPPMs.
These are just two common RPPM examples.
All this means is that if you had been following along and stacking Haste as you got more RPPM items then you will not see as much benefit from that Haste as you do now. The trinkets are half as good and double as good and a little juiced. If you hadn't been stacking Haste before then you probably will notice nothing at all. If you had been then you'll notice a DPS decline.
For more on the RPPM changes check out Lore's posts in this thread.
What can you do? Get one of the new crop of 5.4 trinkets ASAP. They are mind-blowingly good. They seem to go counter to everything we've seen from Blizzard. They give cleave opportunities from non cleave shots. They act as partial readiness abilities. They are generally amazing. Here is our list: