The first thing that comes to my mind when thinking in a Paladin is his ability to stay alive and, above all, help the others.
I can't imagine a Paladin without some healing spell.
One thing that I really like when I'm playing as other class and struggling against a foe, maybe running desperate, and my health bar just go up by a paladin passing by. Sometimes the Paladin will even engage my enemy to tank for me and help me.
What differs from other classes is that a Fighter, a Mage or a Hunter can even help you attacking your enemy trying to help, but by the time you can already be dead. They can't heal you. They can't help with the last 2 HP that you have that if the enemy hits you, you die.
And I really like the feel when I, as a Paladin, can do that for the others.
Another things that come to my mind when I think in a RPG Paladin is their auras, blessings, resistances (against evil, fear, charm) and etc.
Other spells that I can't imagine a Paladin without is Lay on Hands (that drains all you pool as a supreme healing spell) and Smite!