What to consider
Passive bonus A passive intellect bonus is strongest. Depending on your current gear, a passive haste or mastery bonus is also very strong and can be further refined as needed. On the other hand, a passive crit bonus will be much weaker.
Reforging If the trinket is reforgable, that lends you some extra flexibility when attempting to reach hit caps or adapting your gear to different types of content. If I know I'm going to run a lot of heroics, I reforge my hit trinket to haste to make the heroics run more smoothly.
Proc stats Intellect and spellpower procs (or on-use abilities) are the most powerful procs and are most often partnered with haste or mastery passive bonuses. Haste and mastery procs are correspondingly weaker but are paired with a passive intellect bonus and, as such, make for great trinkets. Just please avoid spirit procs, even if you're below hit cap, and crit procs.
On-use vs. on-equip On-use trinket buffs will tend to last for 5 seconds longer than an on-equip trinket but generally have a 2-minute cooldown. On-use is great if you know a fight like the back of your hand and you've picked perfect moments for unleashing all your cooldowns at once to get the maximum benefit possible. However, what will equate to a great boost to burst DPS will not average out as higher DPS over the length of a whole fight. Your choice is between higher average DPS and control. On-equip procs tend to have shorter internal cooldowns and are once less thing to remember in the grand scheme of button pushing. Personally, I prefer on-equip procs.
Ramp-up time Some trinkets have a buff that stacks up over time. Buff stacks may build at a lower rate than expected, and any time spent not casting risks dropping the full stacks off and starting the ramp-up time all over again; however, these trinkets overall often have a very high uptime.
Internal cooldowns and proc chance Unfortunately, neither of these things is obvious in game, so it's worth researching these things from Wowhead before you go looking for a particular drop. If there is no data available from third parties, you can also use an addon like Proculas to help determine uptime, proc chance and internal cooldown.
Trinket league table
These lists were worked out using my own crude spreadsheet and DEP ratings that approximate pre-raid ilevel 346/359 gear, calculated using tools like Binkenstein's ERC spreadsheet, SimulationCraft, and Stelmaria's Elemental Rotation Simulator. If you want to be sure that you're making the right trinket choices, then use those tools with your own gear. Re-sim or re-run the spreadsheets every time you upgrade a piece of gear.
These lists are general comparative rankings, and each successive group is relatively more powerful than the group above. I've made some edits to reflect issues with usage in a practical situation. It is important to think about how the various factors I've described above are combined in each trinket. Also keep in mind what items are accessible for your style of play; just because something is lower on the list doesn't mean it is no good at all. A high-end raider will dismiss Stump of Time, but if you're a very casual raider, it may be perfect for you. Jewelcrafters and alchemists have fantastic profession trinkets for this tier.
Just got to level 85
Talisman of Sinister Order is a great quest trinket and better than most other choices a fresh elemental shaman will have had up until this point. Sorrowsong is actually brilliant for fights with multiple mobs, even at higher levels. The proc is incredibly strong for a very short period of time, but the proc is otherwise worthless for the rest of the right. An item to use with caution, but not to ignore completely. I'm rather looking forward to experimenting with this one, if it ever drops!
Doing heroics and preparing for raids
I do not recommend spending valor points on Soul Casket, as there are plenty of more important upgrades to work through. Anhuur's Hymnal, the heroic version, is a baby version of Stump of Time and well worth grabbing if you're preparing for raids. The heroic versions of Anhuur's Hymnal and Gale of Shadows make a great pair if you're preparing for raids for the first time.
I placed Stump of Time in this bracket because it is brilliant if you are just starting out raiding. As long as you need either the full amount of hit or the remains from reforging, the proc and the chunk of reforged stats make it very attractive. Accordingly, the value drops as your character gets more spirit and hit from other gear. Vibrant Alchemists Stone is not unique and is due to be introduced in patch 4.0.6, and passive stats means it beats out the otherwise very similarly rated Heart of Ignacious.
Best in Show
These hot properties all rank very closely together, with value depending on your own gear and your ability to use cooldowns correctly and at the right moment. There is no getting away from the fact that DMC:V is currently the best trinket available, as the intellect proc is amazing, it has a short internal cooldown, and the direct damage component is nothing to sneeze at, either. Both the heroic and normal versions of Theralion's Mirror are very strong. Heart of Ignacious seems tailor-made for elemental shaman, but the reliance on-use inevitably means it loses out on averages. Figurine - Jeweled Serpent ranks very well alongside the epic trinkets, giving jewelcrafters an DPS early boost.
A few notes
At the moment, the top combo is likely to be Darkmoon Card: Volcano, paired with the heroic version of Theralion's Mirror. DMC:V is likely to be extremely expensive, but it is getting fairly common for guilds to set up card exchanges to help members assemble their trinkets. I prefer trinkets with an on-equip bonus, because I feel it is all too easy to end up "saving" an on-use trinket for the right moment. Very easy to save it too much and hurt your overall DPS as a consequence. However, there is a lot to be said when learning a fight for saving cooldowns for that special burn phase.
Patch 4.0.6 is only likely to increase the value of mastery for elemental shaman, so personally I do not think Theralion's Mirror is going to drop in value when compared to Heart of Ignacious. As with the pre-raid gear, you can see that it is possible to get very strong trinkets from a variety of sources, with 5-mans, Darkmoon Faire, valor points, professions and Tol Barad giving us many possibilities if we're unlucky with a particular drop.
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