What no flying while leveling does, regardless of opinions is this:
1) Makes the world dangerous again. You have to pay attention to your surroundings, watch where you are going. This is a large part of what makes it adventurous whether you recognize it or not.
2) Forces you to learn the land. You use your map more, look at your surroundings more, become familiar with the zones more. This is what exploration is. It doesn't matter whether you enjoy it, that's what it is about.
3) On PvP servers (which I don't like) it makes other players dangerous again, which makes the world more alive.
4) Makes people going for server firsts for leveling and professions on a more even ground.
There are more intricate ways of explaining it, but it's a good thing they don't allow flying off the bat. I personally think I enjoy the game with flying as an addition, but really want a more increased speed at this point on the ground and in the air.