I think I'm finally able to pinpoint why I think these maps suck.
Think back to a map like Highrise. Think of all the less-obvious routes you could take: climbing the cranes and I-beams, getting into the upper windows of the north building, getting all the way to the roof of the south building, sniping from the very top of the crane's support beams, etc.
Now try to think of anything even remotely approaching that in MW3's maps. There isn't any. Every map is a series of tight, narrow corridors leading to a central box intended for three- or four-way firefights.
A perfect example is on Interchange, already mentioned as one of the worst maps ever. There's a tractor-trailer truck, with a huge gap between the cab and the trailer. It seems like an obvious place to jump through for a shortcut to the other side. But nope, invisible wall! "You will follow the exact path we laid out for you." In Outpost, there's a hanger with the midsection of a plane hanging up, and a box in front of it that looks like it was made specifically for you to get into the plane. Nope, can't climb it. It's strictly there to hide behind.
My favorite map in MW3 is any theoretical MW2 map they'll sell us as DLC.