Diego and Mike don't seem to be nearly as impressed as I am, but Halo 2 not only works well on the 360, but looks and works better than on the original XBox. Thinking about how vastly different the two platforms are (Pentium 3 vs. PowerPC w/3 cores) and how much "tweaking" went into Halo 2 to make it so much nicer than Halo (released earlier, and not as nice) I'm pretty flabbergasted.
Here's my reasoning about why I'm so surprised: In theory, the reason that Halo 2 can look good and run so smoothly on the original XBox - which remember is five year old hardware - is because the game developers have had time to learn about the system and really optimize for the hardware underneath. It's commonly held that launch titles aren't ever as nice as titles released later when the developers have more time to really tweak the systems. This happened on the the PS2 as well - and I've seen reports that the new "slimline" version of the PS2 is having problems with some games. They changed the underlying system ever so slightly to get it into a smaller package (probably combining chips, etc.) and it broke a bunch of games. And that's on the same exact platform.
Now think about the move from x86 to PowerPC... that's a massive change. And it's obviously why there's only a few hundred games that are supported right now for backwards compatibility, and nothing released within the past several months is on it. But still. The fact that it works at all is amazing, but working so well? It's seems like something massive to me. (Hey... I wonder if Microsoft's purchase of VirtualPC had anything to do with this stuff? Hmmm.)
Anyways... Those guys said that "API Mapping" would do much of the work, and that makes sense. A basic emulator which convinces the original x86 app that it's running on native hardware, and then something below that maps the old API calls to the new hardware system directly. I'm sure that's a big part of it. But what about the tweaks? The places where the developers were like, "I'm goign to write to this memory address, and do some funky thing to get around this funky bug in the hardware, which will make x and y exponentially faster. That's the sort of thing that I'm thinking about. For that stuff to work, they would've had to perfectly emulate the original XBox and memory, flaws and all.
Anyways, I'll probably write more later about this, but the killer app of the 360 is the same killer app of the original: Halo. It's just fun and works just like it should, and now it looks better than ever. Cool stuff.