Ghostcrawler stopped by the beta mage forums the other day in order to address the issue of resists and immunities on the part of certain bosses. The problem here is that are sometimes special bosses in one raid instance or another that require players to temporarily put aside all the normal gear they've been working so hard to get and put on special gear just for that fight. For example if the boss is a fire elemental, then perhaps they all have to get a separate set of gear with fire resistance on it, and put it on when they get to that boss -- without it they don't have a chance of success. For other boss fights, they may find that the boss is immune to one type of damage or another, and this requires them to respec to a different talent build, or else play a supportive role whereas before they might have been the star damage dealer.
There are clear reasons why many players don't like these mechanics. Who wants to go to all the trouble of collecting a whole set of gear that you use only for one fight? Who wants to let that gear take up all that bag and bank space? Who wants to be shut out of their favorite talent build or sit somewhat on the sidelines just because the bosses they're fighting are immune to their prefered style of damage?
So Ghostcrawler comes in with the developer's logic on this issue: They don't want the raid instance to be a simple thing where you just move from one boss to the next boss, to the next, and so on. They want to break it up a bit so that different bosses require not only different strategies, but different gear, and different abilities, too. This adds a bit of anticipation, of having to get ready for the challenge rather than just stumbling into it and accidentally getting it right on the third or fourth try.