It seems to me, so far, that mining is the least effective use of your time. Copper, coal, iron are common enough, and silver too if you go deeper.
Gold is so rare that crafting anything with it is a horrible timesink.
On the other hand, if you stick to killing the surface mobs over and over, you'll get enough drops to make stuff that's better than metal gear (Snaaaaaaaaaaaake!), and takes much, much less time and tedium to craft.
I haven't found any mini bosses yet. Where do these live?
The other observation I have is about Tier 2 (beta sector). Here I can hit level 14 creatures for 100 damage and kill them in one hit. They are also hitting me for 50 damage and killing me in two hits. My armour is the Snow armour, with a value of 13 on each piece.
Weapons are easy to come by, but armour to keep you alive is not.