1) Its hard to say because most dev teams aren't going to answer that question directly. We don't know the entirety of their vision and there could be things players really want that the dev team intends to put time into later. They've definitely taken time to balance things and keep on top of bugs at the very least. . .most of which they couldn't do without at least listening to what people are saying.

2) I'm sure a lot of what they play is more 'play testing' than just playing the game. When your work day consists of building a game you may want to step back and play something else when you get home. However, given the scope of Valheim and the love we can see for it, I imagine they do take a few minutes here and there to just enjoy it.

3) I think its been stated that optimization is something they intend on working on once the game is finished. I don't have a source for that, but I thought that that had been stated somewhere.

4) Its been the little things. The Sunrise coming up over the meadows. Just the freedom to build something awesome. The enjoyment of getting people together and letting everyone do what they enjoy and still being able to progress. It's also just a pretty game for its simplicity.