In the current iteration here are the things that I see as different from other TD games I've played:
Multiple defense points. You not only have to defend your base but also your metal generators. This seems to have some nice map creation options where you can have a large flow of metal at the start to help get the initial build done, but then threaten it later. It seems like being able to build/rebuild mines could be an option.
Scavenger Modules: Needing to build these to harvest resources adds a twist.
Darkness: Not knowing the full map at the start seems different.
Huge maps; Not being able to see the whole map at once is different
Mission Goals: Nice to have some goals beyond just survival. Although I don't currently see what the reward for accomplishing them is.
Graphics Quality: The look and feel is superior to most TD games I've seen.
These are all fairly minor twists, nothing that will totally redefine the genre. But they give the game a personality.
The things that will make me feel like I got my money's worth are:
How does the gameplay evolve? It won't be long before I have access to all the towers and abilities. So how are subsequent planets different? What makes me want to keep playing?
Amount of content: If a world has 12 - 16 encounters, if there are just the two worlds that are shown now, I'd feel a little ripped off. I'd expect maybe 10 worlds when the product shipped?