Trinkets are one of the trickiest slots to itemize. They have always offered strange and often incredibly powerful abilities so choosing which items to equip there is a subject worthy of some consideration and strategy.
DPS SpellcastersCasters have a bunch of decent options depending on the stats that they value. Like most DPS classes the stats that each caster class wants to stack will vary. The only ones that don't vary a ton are Intellect and Spell Power. Luckily there are some really awesome trinkets that have both of these in spades:
Darkmoon Card: Volcano -- This is a very solid card for any caster all the way up to the heroic raiding level. It's got a big brick of mastery rating, but more importantly has one of the most broken procs in the game currently. The proc currently give " a chance to deal 900 to 1500 additional Fire damage to the target and gain 1600 Intellect for 12 sec." Absolutely crazy. This item is well worth the substantial amount of gold you'll need to purchase it.
Stump of Time -- This trinket doesn't have any special effects or stacking buffs just hit rating and a big wad of spellpower with a decent proc duration and fairly low internal cooldown. It's hard to go wrong with hit or spellpower as a DPS caster so it's definitely worth your time to grind up to exalted with Hellscream's reach to pick up this awesome item.
Itemizing for healing requires you to balance your mana regeneration with your raw healing throughput via spell power. This generally means striking a balance between Spirit and Intellect. When you can grab an item that gives both of these things then, well, you're going to want to pick that up for sure.
Darkmoon Card: Tsunami -- This card along with Darkmoon: Volcano are the "broken" Darkmoon cards of this expansion rivaling the legendary Greatness deck of Wrath of the Lich King. 321 passive intellect paired with a stacking 400 spirit buff make this card absolutely monstrous. This is a must have for any healing class.
Core of Ripeness -- Most of the Valor Point trinkets are really nice, this one is no exception. 321 passive intellect and a strong on-use spirit buff are great for any healers. On-use spirit is particularly strong when paired with any effect that amplifies your base mana regeneration meaning that in the right hands this is a very potent trinket.
Agility has seen some major loving this expansion. It now gives more Attack Power than previously and maintains its contribution to Critical Strike rating. Any agility user is going to be looking for whichever trinkets add big fat bricks of agility to their stats.
Tia's Grace -- As a general rule of thumb any trinket that has a stacking buff of your class' primary stat is a damn good trinket. Tia's grace is no exception. It's got mastery which is so-so depending on your class, but the agility speaks for itself. 300 passive agility is just too good to pass up. Pick up this trinket if you can, it will likely last quite a while.
Fluid Death -- This trinket is unreal. It is the second best Rogue trinket including 372 heroic trinkets next to the trinket available from Heroic Nefarian. Tons of hit rating and a quick-stacking agility buff make this number one on the list to be picked up for any agility using DPS class.
Strength classes have a bit more to consider in Cataclysm than in Wrath of the Lich King. Haste has been somewhat improved to offer more value to Strength stacking classes and the new Mastery stat is always a factor. However, hit and strength are still the big must-haves. There are some ridiculous pre-raid trinkets that let you stack these stats in droves:
Right Eye of Rajh -- This trinket is one of the better Heroic dungeon drops available for any class. It's got a ton of passive hit on it which is always great and it has a strength proc that gives you a beastly 1710 strength for 10 seconds. This is hands down the best trinket available below item level 359 for strength users.
License to Slay -- Like the Agility equivalent, Fluid Death, this is a great trinket and should be one of the first Valor Point purchases for any strength user. With the premium on hit rating in Cataclysm you're never going to have enough and this thing has a big chunk of it. The stacking strength proc is fabulous and makes this trinket a choice piece all the way into Heroic raids.
Itemizing for tanking is really tricky. There isn't really a set of hard and fast rules to adhere to. Each tank has their own style and outside of "lol get more stamina" there is a lot of flexibility for itemization depending on how the tank wants to play. As such there is a lot of flexibility in terms of trinket use. Take these selections with a grain of salt since there really aren't any amazing tanking trinkets outside of raids you have a lot more viable options.
Mirror of Broken Images -- This monster trinket comes from Hellscream's Reach reputation. The passive mastery on it is just "OK" but the on-use ability that buffs magic resistance by 400 is invaluable in any number of PvE encounters. This is an absolutely great trinket that will be useful long into the future on fights that demand anti-magic cooldowns.
Porcelain Crab -- When it comes to Heroic dungeon loot it's hard to go wrong as a tank with the Porcelain Crab. This thing gives a ton of passive dodge and a 20 second Mastery proc. Unfortunately there just aren't that many ridiculous tanking items outside of raids so this is pretty much as good as it gets.