Err God of War 3?  Gran Turismo 5? Heavy Rain?  MAG? 

   As far as I can tell, just like in 2009, all the big quality exclusives are PS3 only (unlike the 360 "exclusives" which have a bad habbit of leaking over to the PC, PS3 or both the PS3 exclusives stay exclusive).  

   Other then yet another Halo game and what looks like a moderately good adventure game (alan wake) I can't see anything next year that looks particularly great thats exclusive just like I can't think of any really good 2009 360 exclusives other then maybe "soon to be overshadowed by GT5" game Forza 3.  Everything good I can get on PC or PS3 just like pretty much every year with the 360.  Now that you can put together a high settings using AAA game playing PC for about 500 bucks thanks to the i5 and mini motherboards that use it I don't see any real advantage to having a 360 these days.

I don't know if there's any system sellers but there's:

Halo Reach
Splinter Cell Conviction
Mass Effect 2 (don't know when it comes out)

I'm sure there's others, thats just off the top of my noggin.

Gears of War 3 will probably be released in 2010 too, if the previous schedule is anything to go by. I wonder if they'll do GOW and Halo in the same year though?

If GOW3 does release, I think it'll be among Halo Reach and GT5 as being the top selling HD exclusives released next year. For Wii, Mario Galaxy and Zelda will be up there too. On DS there's also Pokemon HG/SS. All 1st party stuff interestingly.

PS3 seems to have some good system sellers, GT5 and FFXIII, I think that M.A.G., Heavy Rain, and God of War III could possibly push some systems. Also Sony does have their PS motion wand coming out near spring. So I guess Sony is putting all their eggs in the basket this Q1/Q2.

as for MS, Natal seems to be it's system seller, maybe Alan Wake. I don't think Halo Reach will be a system seller(IMO) if you are dying to play Halo, and haven't already got a 360 after Halo 3, Wars, and ODST. I don't know what's keeping you. I think MS is good at selling software, not harware, unless it's bundled

OFF-TOPIC: I think sony is going to try to destroy the 360 in sales earlier in the year to make up for the big NATAL release that microsoft is planning. (The Wand and several big exclusives hitting all the way up to summer.)

Sony is on the offensive in the first half of the year, then switch to defense mode as microsoft turns on its offense with NATAL coming in xmas 2010.

ON-TOPIC: Yes, There will still be some form of "system selling" power with the games that people mentioned so far, just not as BIG as people like YOU make it out to be. =P

M$ has price cuts. Those are system sellers

Alan Wake is no more a system seller than Heavy Rain is. Just because a game looks good does not mean that it will sell systems. GT5, Halo 3, FF XII, GTAIV, etc. are system sellers. Alan Wake is not.

I alway thought that Halo Reach would be a system seller but after reading what you wrote, I am starting to have my doubt. I mean people who are planning to buy Halo Reach must probably already own a Xbox360.

For the same reason I don't think that  MAG will be a system seller because everyone that is looking foward to play MAG are people who enjoy playing MW2, KZ2 and other FPS online, meaning people who already have a PS3.

I don't think you can count out Microsoft's marketing machine after what they've done for Gears.  Remedy has a great rep too (much better than Quantic Dream frankly), the game could easily take off if it critically hits the mark.  I dunno about "system seller", but in reality there's precious few of those (GTAIV wasn't one even really).

There's always Halo again...

Maybe this new Halo game will get you to buy the system while the others haven't.