Very fun game. It exploits two very powerful aspects of human psychology: love of collecting and flow. The controls are awkward for about five minutes, then you find yourself forgetting about them and losing hours of your life as you explore the game. There's also the gratifying moment where you realise that your character is actually way, way better than when you started: jumping much higher, hitting much harder, charging faster.
I'll admit, it reminds me a LOT of Level Up!--a good thing, because it's one of my favourite games and I recommend it to anyone who liked this. There's also something you could take from the comparison: neutral NPCs. I'd say that's the main thing you could do to add depth to the world of Arzea; the only one I can think of is the cat, who is impossible to interact with.
You might want to take a look at the medals. I earned all the in-game achievements, including the Completionist one, but am missing six medals, the five you get from collecting fragments, keys and orbs, and the 100% completion one. Several other people have mentioned this, so it's not just something weird about my setup.
Because I'm very sad and it took FOREVER for me to find a couple of the items, I managed to level up to level 100 and was hoping for something special. I suppose the character was virtually a minor deity at that point, but I still hoped there'd be a little something extra at that point! That's not so much a criticism as a compliment to how long this game can capture someone's attention. :)