Retraining
It is now an official rule that a character may retrain feats, abilities, and skills. This may be done when a new level is acquired. This means that a character can replace skill points or replace feats or abilities.
While I assume that in most games, including my own, that some rule for retraining existed as a house rule, this is the first time it has been official. While I would put more restriction on it, such as game time or experience loss, I am nonetheless a fan of the added rule.