Some ability runes also change some costs from Essence to a percentage of Health, which you then supplement with abilities to get health back. More than most characters, the Necromancer feels like it's designed for you to set up these Rube Goldberg-esque chains converting one resource into another, feeding your damage.
It was pretty enjoyable, and I think the D3 team did a good job in setting up several possible play styles.
I'm not sure what the future of Diablo 3 is like, but these packs which introduce a new class are an interesting way to go. For example, adding a Druid shape-shifting class would be great.