Often the first difficulty is actually finding the special assistance number on the airline website, it is always there but not always obvious.
Then there often appears to be differences in what details are required to be provided by the service line, airport staff and aircrew,
See recent threads about travelling with Segway and battery powered medical equipment
A greater degree of consistency across airlines and airports would be helpful
Obviously this is difficult for a single carrier to achieve - attempt to be better than the others but not unnecessarily different