This weekend saw the reappearance of the Challenge of the Gods event. This is one event that I'd only really been peripherally aware of before. I remember accidentally picking up a challenge while in a dungeon with my hunter ranger once and being very confused by what was going on.
This time I actually paid more attention and I now think that it's a pretty neat event. Basically mobs will often drop little items that on looting start a challenge (timed quest) for a certain god. These are always very simple tasks, such as killing five enemies within a minute or killing ten without using a daily power (full list here, though I'm not convinced it's entirely accurate).
I actually found it decent fun. Not all challenges are equally feasible at all times - for example completing a heroic encounter can be trivial on some maps but virtually impossible on others. The ones to do certain things while under half health seem impossible to do if you play a high-level character that never even drops that low or has talents/abilities that cause you to instantly heal up again. But that's okay, as the challenge items are plentiful and you can always ignore any given drop and just wait for the next one.
It's not hugely rewarding (I mostly just kept getting pointless potions), but it's a little thing to mix up your play session, as you find yourself playing just that little bit differently to meet your next challenge.