Desk Driver,
The MD-11 failed to meet the specified range and fuel consumption figures MD were quoting. Several customers were pretty ticked with this and MD went into a program intended to improve the aerodynamics (principally by taking measures for drag reduction) and to improve range. In this they were only partly successful.
I'll try to get more details from my reference library shortly but that's about the gist of the problem. Pity in a way, it made quite a good freighter but even that comes to an end (I think) at the end of this year when the MD-11F line at Boeing closes down.
Fedex have bought up a whole lot of ex-United DC-10-10s and Boeing are converting them to a 2-crew glass cockpit. These hulls will probably clock up near 50 years of flying by the time they go to scrap - they may even outlast some MD-11Fs. MD always did build a strong airframe!