As for the preview version you previously exhibited, this game has very solid boss designs. I was honestly confused why the first boss seemed to be so durable until I realized after dying once how he actually took damage. Very, VERY clever detail there. The second boss as well had a clever game mechanic idea that took me a little longer than I would care to admit to figure out.
That said, I found the main game itself a touch boring and stopped a little after getting the grenades. Maybe I have simply player too many games of this genre and have grown a bit tired of it, but the level itself seemed a little empty. Optional houses rarely felt rewarding to enter unless it just happened to hand an extra health point. Everything else was linear, which isn't inherently bad but it just felt a little flat that's all. Hard to exactly say what I mean.
As for the story, I liked absolutely all of what you had so far, but it simply wasn't enough. Perhaps more of this would have alleviated the emptiness I alluded to earlier. And if you are worried not to dump too much of the main character's story right away, explore further mini-stories of other survivors or deceased in optional areas (like the buildings). It actually felt pretty sad to see the dead skeletons of people desperate to get food. What little story you execute, you execute well. It just need to be more fleshed out.
I didn't finish the game so I can't comment further in it. The art was pretty solid, the backgrounds animations all fit in nicely with the themes and mood of the game, as did the music. I'm not really sure how to critique either so I won't go into detail on them, but my lack of comment is not a mark of it being mediocre. I just focus less on it.