well in Halo 2 we played as arbiter, and that was pretty cool.

It will make very little difference, really. Reach may not end up selling as much as the other games, but it'll do very well. What's more important is whether the game comes out well made.

Not by much, the Halo name is enough by now to sell millions.

I think it'll sell for the online multiplayer, not the storyline.

Halo Wars, as well as the 6 or 7 books do it quite well.

I doubt it'll matter too much. If the game is very similar in content/feel to Halo 3 with simply a different protagonist they may lose a few sales due to lack of Master Chief (he is pretty iconic after all) but I think by now the Halo franchise is decently established to survive without him as the main character.

By no means do I think it will not sell well. I do however believe it will have an impact on sales. How much is left up to conjecture.

Well, Halo Wars did very, very well for a console RTS, and doesn't feature him (at least not in a primary role). That and ODST's sales will be a good indicator, since I don't believe ODST even has Spartans in it.

It won't affect nothing.

Halo Reach will sell 15 million copies.

Not as much as it would it being an "expansion" (ODST) or a different genre "RTS", this is the next "major" Halo game and people will know that. Either way, from the sillohuete it looks like Chief is still there.