I haven't purchsed ODST yet, but I have played it at my cousins for a few hours.  I think its fun and I don't see why people don't like it.  

I am a cheap bastard when it comes to purchasing maps etc...  so I never purchased anything for Halo 3, when I do make the jump over to ODST I will have the added value of all of the multi player maps to go in with it so thats a win win for me.

I think it'll need:
1) A spartan at the forefront of the story (main character)
2) Severely heavy marketing to remind people that it is the next big halo FPS shooter installment, and not a half-step, side-story or different genre game.
3) Some amazing multiplayer with no problems from day 1

If it has all that, it might sell as much as Halo 3. I'd think 15 million is highly unlikely.

This is probably the biggest gaming franchise outside of Mario currently out. If Reach really does make strong use of Natal as a value added perifrial (or comes packed into stand alone with some sport titles of course it could really boost the things sales numbers) and if Microsoft can show why this game in the next installment that every Halo 3 fan needs to jump on board with they'll easily hit 6-7 million before the end of the year... probably closer to 8. Personally I wanted to see Bioware make a Halo game... that would be f'n insane... fucking Microsoft letting that company be taken in by EA... what a bone head move though I'd like to know the politics that went on with that one

What honestly makes people think Halo: Reach would sell 15 mil?

I reckon Halo 3 would reach the 15 mil mark before Reach even reaches 10 mil.

Once Reach is released you can count on Halo 3's sales dropping off a cliff.  It would be like people purchasing Halo 2 when halo 3 was out.  Not really much incentive really because everyone will be in the new game.  ODST never gave anyone the incentive to stop playing h3.

Watch a teaser for Reach

There will be Spartans, not one Spartan but few of them at least.

The story will be a prequel to Halo trilogy.

One milion people every day play Halo 3 on MP after two years.

There will be about 40m X360 on the market when Reach shows up and probably around 55-60 till the end of 2012.

Bungie is working almost two years on a new engine for the game.

ODST still sold buckets even that it was given the tag of being "an expansion that lasts 5h"

Halo 3 is still selling 20-30k every week after two years.

Fair enough but that still doens't give a full justification as to why some people think it'll sell 15 mil, even more than Halo 3. Let's assume that Halo 3 reaches 11.5 mil by the time Reach is released. Even after the release of Reach, Halo 3 continues to sell so it reaches 12 mil. So some people are saying that  Reach will sell more copies than Halo 3 in 2 yrs or how long it ever takes for Reach to sell 15 mil?! That's VERY highly unlikely to happen.

Yes Bungie is developing Reach. Yes they're making it from a new engine. Yes they're probably going to introduce new gameplay. Yes the book might've been good. But all that ain't gonna prove that Reach would sell 15 mil. I truly believe that no 360 game will sell outsell Halo 3. I said this previously in a different topic. Not even a Halo game can kill Halo 3 in sales.

And don't get the impression that I said that because of the way ODST is selling. ODST doesn't even belong in this argument. No one can really say that Reach won't sell a lot because of how ODST is going because most people would know that ODST was just a simple 'expansion' and Reach I presume would turn out to be a full game that's worth the $60.

I've seen the teaser ages ago.

Adding a few more if not a lot more spartans is gonna increase sales?

Do prequels really sell a lot? The fact that it's a prequel gives it another reason why it won't sell as much as Halo 3.

So what if one million play Halo 3 everyday?

So an increased user base is gonna get people buying a prequel to the whole Halo trilogy?

Wow new engine. Any developer can say we've been working on a new engine for ages and that ain't gonna push sales to the max even though it's a Halo game. And definitely no comparison to Killzone 2 here.

Read my previous in regards to ODST.

The fact that Halo 3 is still selling well means that it's gonna be harder for Reach to even match Halo 3's sales.

For me the facts that old Halo 3 is still selling very good after two years and huge number of people still play this game (and so on) are the indicators that Reach will be a sales monster bigger than Halo 3 because there clearly is a need for a Halo franchise and it will grow alongside inrease of the userbase and if M$ takes some serious advertasing Reach can make it to 15m in two years. In 3 years from now there will probably be more 10m sellers on X360 so why not to think that the biggest franchise could brake another barier I don't say it will definitely do it, but it's probable.

You never know mate. It may or may not.

Let's reflect back on this topic in 3 yrs time and lets see whether it does sell 15 mil or not.

Of course feel free to continue speculating but I just firmly believe it won't sell 15 mil. That is all.