This was an expansion pack that was content complete almost a year ago. It's a flagship title because MS wanted to squeeze $60 out of it, not by the intent of the creators. They wanted to release all 6 Mythic maps last fall and ODST in the spring of this year. iirc it was made by a team of like 12 guys.
You're on the market for this if you enjoy Halo to some degree. If you enjoy Halo you got used to the graphics a long time ago.
If Halo Reach still looks like shit it deserves tons of scorn.
Who cares. ODST looks like it plays closer to the original Halo than the other games, which makes me very happy since the original Halo is still the best one. Good game is a good game.
Uh, what's your reasoning behind this? It has an open world and a bunch of non-Spartan stuff, it looks nothing like the original Halo. 
Uh, the 'non-spartans' in this game have the attributes of Master Chief in Halo 1. Fall damage, a health meter, no dual wielding, a scoped pistol.
But you're right, the gameplay scenarios are quite different this time around and that's why the game is worth buying. Firefight is enough for me though.