They want dispels to be less important in pvp in cata. To do this they removed all dispel resistance and classes have a lot less dispel options (3 types each I believe).
Now the most important change regarding dispels in cata is that if you cast for example dispel magic on a enemy but he has no beneficial spells on him, you will still actual cast it and it will cast you mana (and a lot more then now if I have to believe blizzard) and a GCD. As opposed to the current message saying there is simply nothing to dispel. This is to prevent dispel spamming to get rid of things immediately.
In other words they want to make dispels more of a choice and not make it always dispel the second you can.
