The game doesn't feel empty, it doesn't need 'quests' and has enough monsters to deal with considering the deeper you go the faster they spawn.
It really does after you've already done everything in it. Starting out is hard and slow, but once you've gotten your first meteorite set the rest of the game slides downhill pretty fast. Before long you're just farming up souls to summon the uber bosses so you can farm THEIR souls for the excalibur and its associated armor and then...then what? You've got the best gear in the game, you can kill anything and go anywhere, but there's no reason to go anywhere since you can't find anything more you'll need unless you're trying to craft some very specific kind of structure and everywhere you go looks the same.
There was a time when I tried covering an entire map in the faerie ground (hallow?) but that got pretty dull, though it was a little amusing filling in the corrupted sinkholes with pewpew white light for the first couple. The game just has limited replay value.
I'm hopeful Starbound will be better in that regard, but not sure.