As I recall it from the first time through, the second-to-last mission, "Cortana," was particularly difficult and much more difficult than the previous missions. Given how hard Legendary is--doable, but very difficult--I'm curious how that's going to go. I can't say I'm looking forward to it in some ways.
BTW: In addition to the sub-HD graphics in Halo 3, split screen actually reduces the screen real estate even further by giving you two "windows" to play through (one for each player). These windows don't extend to the left and right edges of the screen, so it's like looking through a microscope, sort of. That's terrible, and yet another way in which this game under-delivers. That said, co-op is so good I can almost forgive Bungie for this. But I really need to try co-op across two LAN-based 360s for the full effect.