Downturn has no natural flow to it.
It has no viable choke points.
It's the only map where assassin pro bugs me because of how well everyone blends in. It feels like I was killed by a wall shooting at me most of the time. Even without assassin pro it's a pain in the ass.
The underground portion is a pointless maze.
I've always hated the most popular maps in CoD games because they were clearly meant for an ADHD playstyle. Oddly enough I love Nuketown.