I'm currently testing out a more or less ranged-focused Wizard deck that uses Steep Hill, since it naturally works well with him and some of his creatures. Might be worth building around, not sure yet.
I don't think terrain is overpowered, and I like that it rewards decisive decision-making. Obviously, there should and I assume there will, be terrain of many different school types, but simply increasing your board position is a good way to force your opponent out of an entrenched position. If they spend mana on a terrain that does not adequately benefit them and you spend mana on increasing your channeling or dice count, you're going to have the advantage.
I don't actually see why a Dark mage would be afraid of terrain...with easy access to Idol of Pestilence, Plauged, Malacoda, Altar of Skulls, etc, you aren't hurting for zone/arena-wide damage. And, with Bloodreaper and Zombie strategies, you should be considering ways to deal some initial damage anyway if you can't just do it through attacking.
The Wizard, even though he's only Air school now, can still use Poison Gas Cloud or Mordok's Obelisk.
Mohktari (in general) and Temple of Meraveran (for holy swarm builds) is a solid way to fight on enemy turf.
Elephant Grass can certainly be an issue with how powerful it is, but you can also destroy it, unlike most terrain.