Terraria is for people who like fighting. The main reason for spelunking is to get better weapons/armor/accessories.
That's how I see it.
The way I see it this whole thing started with Dwarf Fortress. Notch took the idea, cut out all the fortress stuff and left the mining, pretty much creating a new genre in the process. Terraria would just be the second game added to that genre. No more a copy cat than Halflife is of Quake, and it used the same basic engine. I expect we'll be seeing more games like this soon. http://fabritoid.blogspot.com/