One thing that seemed immediately obvious (and was already addressed the first time I looked at Neal Stephenson's website) is that the torus at the aft end of Izzy is in the wrong place. It's a big gyroscope, and since Izzy (like many other spacecraft) executes a 360-deg forward flip once per revolution (to stay properly oriented w/r/t Earth), gyroscopic precession would make it simultaneously execute a 360-deg. rotation in yaw. Thus, halfway through each revolution, it would be flying ack-basswards. Eliminating this precession would require constant thruster operation and would impose huge structural loads. Building the second torus (T2 in the novel) and spinning it the other way would help solve the problem--but Izzy and the first torus wouldn't have lasted long enough for that to happen.
Quick and dirty solution: either build two counter-rotating tori right from the start (although the loads on their hubs and spokes would still be significant), or build a counterrotating mass (can be a lot smaller if it's denser and spins faster) on the same axle.
Better idea?: put the axle of the torus (or tori) on Izzy's port/starboard axis so that it/they rotate along the direction of orbital travel--like bicycle wheels. Of course, in any situation in which you have to slew the spacecraft such that the gyro axis is displaced, you'll have to deal with precession.
That's why instruments like Hubble or Kepler have "reaction wheels" to control their attitude in the first place!
