The idea of a haste "goal" has enjoyed an irritating persistence, thanks to higher-end Wrath raiders' reaching the soft haste cap for Lightning Bolt and bringing the cast time below the global cooldown. Even after that break point, at the time there was still a lot of value in the haste stat, as it still affected Lightning Bolt for the rest of the fight and Lava Burst all the time.
Haste thresholds and DoTs
In Cataclysm, haste thresholds (also known as break points) are a hot topic for classes that rely on a lot of HoTs or DoTs. The new HoT/DoT haste mechanic reduces the time between ticks, rather than overall spell duration. Haste thresholds are where enough haste is gained to fit an extra tick into the duration. This idea has been picked up and asked about in the elemental community as well.
For a healer in these mana-strapped times, an extra tick increases efficiency and heals per cast. For a DPS class (DoT-heavy classes aside), this threshold has less value in a live raid environment. As elemental shaman have only one DoT and our other spells continue to benefit from haste even after the soft haste cap, there is no break point beyond which we get to stop thinking about haste due to DoT mechanics. In other words, such haste thresholds are little more than a nice number to let you know that your gear is heading in roughly the right direction.
Haste vs. mastery
In the past, elemental shaman have been in the position of stacking haste and spellpower, doing very little else. This gearing tactic is now problematic due to the increasing value of mastery. The big question of when to stop stacking haste and start picking up mastery tends to focus on the idea of some sort of threshold. The difficulty with stat stacking -- and the reason I have never advocated stacking haste to the exclusion of mastery in the past -- is that haste and mastery scale in a synergistic fashion. If you have a poke around with any sim or spreadsheet and slowly increase the amounts of both stats, it is possible to see the stat weightings/values playing leap-frog with each other.
The root of this synergy is that both stats increase the total amount of Rolling Thunder procs within a given period. Haste does this by decreasing cast time, resulting in more Lightning Bolts cast. Mastery does this by increasing the proc chance per cast. Haste basically creates more opportunities for mastery to get a look-in during a given fight, as well as its basic DPS increase via the reduction of cast times. As a result, both stats result in Lightning Shield's stacking faster. If you have high amounts of both stats? That's a very tasty situation for an elemental shaman, especially if the actual spells pack a hefty punch due to high intellect levels.
Don't take stat rankings at face value
A lot of confusion is created by taking specific stat weights, as generated by sims and spreadsheets, as somewhat static statements of value. This is compounded when a particular set of stat weights is then used with Wowhead filters to choose an upgrade. What works out as the biggest upgrade for one set of gear might actually rank lower for another set of gear. Your 359 raid gear is not the same as my 346 heroic gear. A good best-in-slot list is actually a "best gear compilation," as SimulationCraft and RAWR output a set that creates the best simulated DPS, rather than a list of items compared by stat weight.
Lab conditions vs. field tests
Finally, whenever you sim or spreadsheet a gear set, remember that it is not a perfect model of what really happens when you personally go up against a particular boss. Certain glyphs and stats increase or decrease in value depending on the nature of the fight and your personal ability to maximize DPS.
As far as movement goes, we benefit from mastery due to Rolling Thunder procs, resulting in a higher chance of having a 7+ stack Lightning Shield available while moving. Haste benefits us during movement due to Flame Shock and any time we are casting using Spiritwalker's Grace. Otherwise, intellect is the main boost during movement. For AoE, haste benefits our Chain Lightning, Flame Shock, and Earthquake. Mastery only really assists via Rolling Thunder procs from Chain Lightning, although this matters more once patch 4.1 goes live. ... All things to keep in mind.
How to pick an upgrade
So which piece of gear to chose, and how to reforge it? The min-maxing choice that our enhancement shaman kin have been familiar for a while is to sim everything to see if increased DPS is a result after adding it to your current gear. For many players, the intricacy of the gear puzzle is fun. For others, it is just tiresome. The practical choice for a player who is not interested in simming every piece of gear is to balance mastery and haste. For most players, it isn't possible to get equal percentages of mastery and haste, but just don't be afraid of replacing a haste item with a mastery-based upgrade (e.g. haste/spirit to mastery/haste, with more intellect on the mastery piece).
In a production environment, haste is the choice for a lot of players because it is always a consistent increase to DPS. One reason I'm a personal fan of haste is the tangible impact it has on playing almost immediately. At the same time, don't discount the impact mastery can have. With more mastery, there will come a greater need to juggle shock cooldowns, which is perhaps more fun than casting more Lightning Bolts. Push two buttons more often, or push the new Earth Shock button more often? While Fulmination is a small portion of our damage, more mastery also means more DPS choices in movement-heavy fights. Thus when it comes to a matter of gemming and reforging, there is now a choice beyond ye olde stack haste.
Personally, I like fast cast times, so I stick to the method of using Reckless gems in yellow sockets and reforging crit to haste -- but on the flip side, I am much happier to grab mastery upgrades than I was at the start of Cataclysm. I think Blizzard is doing a good job of making the stat puzzle interesting, while still maintaining a level of obviousness and simplicity for those less interested in the perfect gear set.
Show your totemic mastery by reading Totem Talk. Ready to take your elemental shaman up to the new level cap? We'll help you get to level 85 with our guide to leveling an elemental shaman. Check out our Cataclysm 101 for elemental shaman, or pick up our pre-raid gearing guide.