I agree!

A more dreary environment would really make the scenery better, and set the atmosphere for the game. Not a completely depressing scenery, just believable for a post-apocalyptic setting, maybe just for the cities anyway.
There's nothing wrong with a color and sunshine, maybe a good variable could be implemented, Fog, dark overcast, etc.

Disagree. Imho, an environment is more creepy when it looks like everyday with usual colors and weather, without obvious traces of war, acid yellow rains or fogs... I acknowledge it's a question of personal tastes, so i would play a game which puts me in a simple tomorrow's day, when all people suddenly disappear. Without hackneyed stories about viruses, alien invasion, war between several political sides, nuclear bombs etc. Without a clear story the game will stay more universal (even if it has some official explanation, for the moment its environment lets us to not be involved in some concrete scenario). Even if you just add some signs of anarchy and chaos such as bigger destructions or writings on the walls, it will make a story and questions - such as "where are all these people who left so many traces if there are just 40 survivors on the whole map and few zombies in every town?"

I didn't play other apocalypses games, but we all can see releases of recent years, so i bet now being so "usual" its environment stays more unique. Shouldn't be changed for look like "one of those...", which you saw in movies or read in books. While the game is so simple, it stays rich.