I don't know if there are any MD-11 pilots on this forum, but from what I have heard from some airline pilots is that the MD-11 is a real b*tch of an aircraft to fly with overly complex systems. Most of the instability stems from what David pointed out, the horizontal tailplane being too small and the pitch instability most apparent at low speeds in approach. Supposedly (and again, I'm open to any differing opinions on this) it's also a real dog when manually flown at cruise as well.
There are some who are of the opinion that this is a critical design flaw in the MD-11 that's likely had a role in several accidents already in which pilots overcontrolled the aircraft thanks to its aerodynamics which shift the CoG aft to reduce drag and fuel burn and exacerbate the problems caused by the smaller-than-ideal horizontal tailplane.
Piss on noise abatement!