Without resorting to game terms, or specific mechanics, each class can be summed up in an easy-to-understand sentence, which contains the basic nature of the class.
The hunter has a pet animal, and shoots enemies with a bow or gun. The rogue sneaks around, and stabs enemies in the back. The mage blasts enemies with raw elemental or magical power. The warlock summons demons, curses enemies, steals souls, and generally trafficks in all manner of darkness.
When elements of this basic essence of the class disappear from the playstyle, it leads to unhappiness. For example, I believe that hunters are happier now that they can use their pets in instances.
What is the basic nature of the paladin?
The paladin heals her allies, and smites her enemies with a giant hammer.
Both elements are necessary. Just healing is not enough, and just smiting is not enough. A paladin is both shield AND hammer.
In raid content, Holy fails this test, but Retribution fails just as badly. Respeccing to a warrior-lite is not enough, but neither is speccing to be a priest-lite. I enjoyed playing a paladin while levelling. Now that I am raiding, I would like to play a paladin as a paladin once again.
That's all. I don't want to top the DPS charts. I don't want to top the healing healing charts. All I want is to do is heal my allies and smite my enemies with a giant hammer, without being a liability to my raid.
And for those of you saying reroll, I chose the paladin for a reason. If I had wanted to only do damage, I would have rolled a warrior or rogue. If I had wanted to only heal, I would rolled a priest or druid.