IMO, I think Gears was just at the right place at the right time under the right conditions.
Gears had major post E3-2006 (basically MS's #1 game, in-game gameplay made critics/reviewers say it was going to trump Halo 3). This certainly gave the game huge buzz, as everyone was just raving for the next 6 months.
Then when it came, it had very weak competition against RSV and CoD3, and was certainly the stand out title and the must-have title for the 360 of November and December. Gears didn't have insane 1st week sales. It took it about 2 weeks to hit 1m, but 2m and 3m quickly fell once word got out that the game was actually as good as everyone said, and hit 3m by December. Gears hasn't been, or wasn't frontloaded like other blockbuster games such as Halo.
Not only this, all of the other uber-360 titles have done well, but most of them (IMO) were early on in the 360 lifespan, or very US-centered games. TPS's are very popular in Europe, and Gears has, or nearly has, outsold Halo 2 there.
Having said this, it clearly shows that Gears dominated so well considering Gears has basically locked #1 on MGC since November. I check the site regularly, and only Forza and Crackdown have beat Gears for 1 week each since Gears came out. Despite this, we can see that games like RSV are doing well, since they aren't selling in the shadows of Gears anymore.
Going back to Gears, it sold around 1.1m units in November (I won the GoW prediction league back when it started). Then dropped to 850k-ish in December (again, very good), sold around 250~300k in January, and has been selling well since. Any game that can sell 1m copies in a month + nearly another 1m in another month (even in December. FFXII sold nearly the same in November, but dropped to 300k or so in Dec.).
Games that'll beat Gears? Halo 3 will, obviously. Outside of this, I really don't see anything upcoming that'll beat Gears, as it sold on the Halo-beater vibe. I do think, however, that Mass Effect and Lost Odyssey have a great chance, as both should sell real well outside the US, and if they did good here, would have a good chance at getting near 3m each.
That's not to say that other 360 games haven't had great runs. Call of Duty 2 was a beast when it came out, and sold to 70% of every 360 owner @ launch (I was one of the few that didn't buy it at launch), and kept a 70% attach ratio for around 4 months. It sold 100k+ every month for nearly 6 months, and sold well for a whole year. FNR3 is obviously great, and GRAW + Oblivion have both had great runs. RSV continues to have great holds, and now Forza and Guitar Hero 2 have chances of breaking the 2m barrier.
IMO, at the end of the year, the 2m barrier should be shattered by quite a few games. H3, Madden 08, Mass Effect, Forza 2, Guitar Hero 2, Blue Dragon and Lost Odyssey all have great chances of breaking the 2m barrier. Madden and H3 could do 2m+ in the US alone (Madden 07 had 1.6m sales in the US, an improvement of 1m copies vs. Madden 06).
Back from the dead, I'm afraid.
