Long ago, when I was still but a wee lad in the MMO world (and real world, sort of) a game I played a lot made a drastic change to one of its core systems. Namely, EQ2 changed the way leveling classes worked.
Originally it was much like this game: You started as an Archetype: Fight, Mage, Scout, or Priest, then picked a class that branched off from each of those at lvl 10, then a subclass at 20.
When they changed it, they made it so you started as the subclasses. At the time I was appalled, and even quit the game.
Well, years later I came back and saw the wisdom in the system: By nixing the old system, it gave the developers more freedom to add new newbie zones and not have the players hamstrung by a series of quests they HAD to do. It also let them add more classes as well as diversify the existing classes more before lvl 20, allowing them to be less homogeneous.
This was a pretty drastic choice on their part, but it illustrates a point valid here: Sometimes what you have planned, especially in the MMO world, isn't always possible or practical with the systems you're given. Every announcement an MMO developer makes for something should be taken with a grain of salt. This is what they WANT to do, what they PLAN to do, but it's not always practical.
And let's not even get into some of the promises made by game devs for games like SWG or Shadowbane...