If you're rolling a new thief, they unlock pilfering at Character Level (CL) 3. Should be easy to get there. Try disguise, hide, punch something in the face, stab them in the throat, sneak away and you're probably there.
For existing thieves, pilfering was based on the difference in level between you and your target, with a cap equal to your overall character level. There was a little going on as far as being discovered that involved your character level and wisdom(?!) but that's about it as far as complexity. Most thieves after a certain point could grab all an npc had in one swipe, which is hardly effective for training the skill. I had an idea recently for how to handle the amount being stolen (in the middle of class of all places), so pilfering got bumped up the list for its overhaul.
For starters, the stamina cost has been increased from 2 to 4 and the delay to use has been lowered to 1 second. This is inline with an overall theme of raising stamina cost and lowering delays to most skills and spells. Next, there is now a racial bonus for pilfering success with goblins being the best, followed by bolinaus, humans, vukasins, and elves. The amount stolen is now a percentage based on level difference between you and your target, your pilfering skill level, and your dexterity. These factors also determine the maximum amount you can swipe in one attempt. There's considerable bonuses for being hidden, as well as penalties to your chances of being seen if not hidden. Lastly, some cosmetic changes were made to the display text (wording, color, etc).
Also in recent months we went through all the towns and cities and updated the npcs that inhabit them. All are more pilfer friendly with replenishing coins over time and less severe memories of that one time you mugged them three weeks ago (let it go already). Some pilferable townies were added to the academy sewers, plus the harbor townies and student around the academy were all updated as well. Pearl has specific areas set up to favor thieves like places around King's Port, and then smaller areas like Rum Village and Three Bridges have also been upgraded to better support a life of thievery.
I think thieves have seen some good advancements in the last year. I'd love to hear what you think.