On offense: I watch the Oline, and often rewind during commercials to check something out. As someone else pointed out Iupati is a monster in the run game, absolute beast, often climbs to the 2nd level and mauls dudes. Only other thing I watch on offense is where Fitz is lined up, whether he's in motion, and whether or not he crashes the line to block or runs a route. I've become adept at deciphering runs whether Fitz is in motion or not. I've gotten so good my wife actually asked last game how I know and when I explained she was calling them out also. That's not a good thing, but nobody's been able to stop us, so who cares.
On defense: I always watch the LB's and try and figure out where HB is. Again, as someone else pointed out, you don't really have to key on HB, he's always in the frame on TV around the ball, every time. His natural instincts and range are sickening. Our LB's are so bad, I just haven't harped on them because it's rather obvious, but when you watch them, they literally (no exaggeration) look like they try to find an offensive linemen to engage. Very few stunts, loops, etc, it's just straight ahead, find a linemen and engage. It's by design, Bettcher must believe 1 on 1 matchups are good and expect them to beat their man, but they rarely do, they just crash, engage, and try to shed. Rarely works.