Basically, I'm going to have to be the cynic here and slap the "dark horse" label on NOVA. Their "About" page doesn't really give me a lot of confidence in their plans.
What are you guys trying to achieve by building yet another abstraction layer? Why not work with the Sponge guys? Sponge already has plans for client-side modding as well.
My argument against yet another abstraction layer is that there will be the inevitable people who code very deeply into the innards of Minecraft, thus defeating the point of an abstraction layer because "what for, when your mod only works on one implementation?" Also, there will still be people who will try to run Minecraft mods on Terasology, for example, and NOVA will make it only easier. Are you prepared to deal with the disjointedness, the "it's not supposed to be there" feeling and the differences in architectures and design principles between voxel games? Are you prepared to redesign your mod 5 times because it's perfectly balanced in Minetest but hopelessly OP in Minecraft and in need of severe buffing in Minia (or whatever Eloraam calls what she's working on)?
I fear that switching to primarily using an abstraction layer will be detrimental towards Minecraft modding. Look at Bukkit, for example. All the people reflecting into net.minecraft.server because they want to do Minecraft-specific things which the Bukkit API didn't allow. And when bukkit broke that... The shitstorm, I don't even want to remember. Or more recently, when Emperor LexManos in a fit of rage threatened to ban coremods... How did everyone react?
Having a low-level hook layer like what Forge already is would be better. NOVA would serve a considerably different purpose from Forge, the way I look at it. Forge needs to be replaced (or improved, but is that ever going to happen under current management?), not abstracted away with yet another abstraction layer.
Just my 2 cents.