On the forums we have a rep system. It has been met with mixed reviews of sorts, but it serves a purpose. At the very least it shows who has contributed to the community in one form or fashion (even if it is just adding some levity and humor), and it isn't really worth the time to abuse. I think a rep system in-game would be far more useful as far more players actually spend time in game.
It doesn't have to be an overbearing sword-of-damocles kind of thing. Maybe something you can see by clicking "examine" on someone or by right-clicking a toon's name on the chat. But a positive point could be rewarded by players for any reason, successful completion of a quest, useful advice, being a good old chap, whatever. A negative point could be applied by those players who have earned a high rep to those players who contribute in a negative environment in the game or are considered griefers. Only one point per account would be allowed, and some reasonable limitation on accounts should be considered. Perhaps free-to-play accounts wouldn't be allowed to add negative rep. Perhaps only premium and VIP accounts would be allowed to add rep at all, or maybe theirs would count for more (although I could see some pitfalls to the latter).
An in-game rep system wouldn't have any ramifications beyond those the player viewing it wanted it to have. It would only be positive and negative points. Points would apply universally to the entire account. Like eBay, players would be able to see a % positive rep rating if they wished, but it wouldn't have any in-game mechanical effect. Most importantly it would show new players which players are worth paying attention to when advice is solicited. When Barkilar with a rep score of 7 (85% positive) says to buy the Catacombs adventure pack while TombRaider with a rep score of 415 (99% positive) says to buy Delara's, the player asking which pack to buy would have a good idea of which player is more likely to give better advice. Encouraging good advice encourages player retention, which is good for the game. And the players don't have to visit the forums to find out (which most don't do anyway).