Wrath of the Lich King suffered from), and sometimes the feature in question just winds up getting lost in the shuffle. At other times, the system is in the game -- but less cool than you imagined, even though it's functionally exactly what you were told it would be.
You might have been really looking forward to making the moral jump in City of Heroes, but when Going Rogue went live you were disappointed by the fact that shifting from rogue to hero (or vigilante to villain) is a notable jump. You might have really looked forward to Final Fantasy XIV's class-blending, only to find that the combat classes and the gathering classes are firmly segregated. It doesn't make a cool game bad, but it means that one of the things you were anticipating isn't there after all. What cool game feature have you heard about only to find that in practice, it's not quite as great as you hoped?