One of the quirks to Dungeons and Dragons is that the game system has always had a class (currently called the rogue) who specializes in opening locks and disarming the traps found in dungeons. From a design perspective, this raises a bit of a chicken and egg problem - if your dungeons does not have traps, there's no need for a rogue, but is it really fun to tell a group they're flat out screwed if they don't have a rogue but the dungeon is packed full of traps?
The workaround for this in the traditional tabletop game is that players know who they're playing with and what classes they're intending to play - effectively, someone can step forward to play the party rogue. In an online game where you're playing with strangers, that plan runs into a bit of a snag - it's hard enough dictating that there has to be a tank and a healer (however your game of choice handles those roles) without adding in an additional role that is sometimes essential and sometimes useless.
So how does Turbine attempt to address this problem? Incentives.
As with the pen and paper game, DDO does not hand out experience immediately when players kill mobs, advance quests, etc. Rather, an exp number is doled out when players complete the final objective of the dungeon - killing all the mobs, or sneaking past them are both valid ways of approaching the problem. However, in this model, sneaking past mobs would always win - less risk of being injured in combat, less resources/consumables spent, and less total time per quest leading to higher exp.
So, DDO modifies the base experience number in a variety of ways. Killing most of the enemies in the dungeon gets a bonus. Completing the dungeon without dying gets a bonus (part death penalty and part insurance policy so that players don't use suicide tactics to pick off foes in pursuit of the killing bonus). And, as you see in the screenshot above, there are bonuses for uncovering hidden doors and disarming traps.
I have no idea how well this system works out in social practice, but it's a fascinating principle. It means that the traps in the dungeon can be optional, but players will still want to bring a rogue to obtain an across the board experience boost. Almost all DDO quests can be repeated (and sometimes must for sufficient exp gain), but the exp award drops with each repeat, so every bit of exp potentially matters, especially for players trying to get by at higher levels with the relatively more limited selection of free to play quests.
Of course, a minor side effect of this plan is that many players choose to add a single level or two of rogue to their existing builds so that they can serve as a "trapmonkey" while still filling whatever their normal class role would have been almost as well. Sometimes there's no keeping us pesky players in line. :)