Any caster worth their salt can remember the frustration involved in trying to nab the elusive Illustration of the Dragon Soul trinket from Sartharion in the Obsidian Sanctum. We tried everything we could to get it, from creating our own melee-heavy raid groups, to participating only in "3D zerg" runs hoping that participants would be too over-geared to want it, to participating in Less Is More achievement runs just so there would be less competition for it.
Towards the end of the Patch 3.2 days, people would run OS25 once a week just to try and get that one trinket. Most weeks, it wouldn't even drop. When it did, you'd have to beat a good twelve or thirteen people in a roll for it.
The Illustration was so valuable (1) because it was a darn good trinket with gobs of spell power, and (2) it seemed impossible get it -- or any good trinket, for that matter. You can still head to Dalaran and buy trinkets with emblems, like the Talisman of Resurgence, but their use is painfully specialized and their value is often less than trinkets you can find in lower level heroics.
It turns out that good trinkets are hard to find for a reason: Blizzard like it that way. Bornakk at the official World of Warcraft forums wrote today:
"Trinkets are really the one item (weapons are to a lesser extent) that we use to try and capture that old slot machine feel from the classic Molten Core days. Yes, completely random loot with huge loot tables can be frustrating but we are pretty far from being completely random these days. Trinkets however are still somewhat random, they can be hard to acquire, and there is often a lot of competition for them."