Enhanced Original is a term used to describe the type of vampire Alaric Saltzman became when Esther recreated the original vampire spell to turn him. She made changes such as binding Alaric's life to the doppelgänger whereas with the other Originals their Immortality was given from the White Oak Tree. This means that he cannot be killed at all as long as Elena lives, thus meaning even the White Oak Stake or the daggers have no effect on him at all as it will not even neutralize him. The only way he could die is if someone killed Elena, which is why he had to guard her life. His darker personality is enhanced as well, as his original personality is now gone completely, replaced by the vampire-hating serial killer that was created as a result of using the Gilbert ring to come back to life.
Whether Originals were always as strong as they are now or simply were once weaker but grew stronger through age is unknown, but Alaric, a newborn Original, was easily stronger than Caroline, Stefan, Damon, Rebekah, and even Klaus, the latter two being at least 1,000 years old. This is demonstrated when Alaric easily removes Klaus's hand from his chest and twists it and proceeds to throw him around as if he were nothing, and almost staking him, though Elena saved Klaus at that time by threatening to kill herself. He again easily overpowers Rebekah in The Departed.
Since the Original's immense power comes from the spell being designed to make them stronger and faster than the werewolves, it is then likely that the Enhanced Original is superior in every way to normal Originals from strength to speed because he is designed to kill other Originals by Esther, thus producing the "Ultimate Hunter", someone with enough power to wipe out all of the original family left living.
His life is bound to Elena's, as Esther needed a Doppelgänger since they have a limited human life span, as long as Elena stays alive he will too and it is also a way of ensuring he will eventually die as Esther does not intend to have any more vampires roaming around after the Originals and their progeny have been killed. If Elena was near death or dying, then it would affect Alaric. When Elena was dying as Alaric is fighting Damon, it allowed Damon to get the upper hand, and Alaric died as soon as Elena drowned underwater.