Oh well...
Have a FAA ATP, but no JAA licence yet. This is due to change though.
Difference is in the systems not in the licenceing itself. Where US system is based in gaining experience and moving up the ladder from smaller airplanes to the big commercial jets. Thus initial training does not need to be very deep and thorough. As for EU, in most countries training has prepared a pilot for a direct entry position in a large jet. Does this preparation make a pilot ready for the challenge ? sometimes yes and sometimes NO.
So both systems have their goods and bads. My personal frustration is the lack of information on the matter of obtaining a JAA licence and still getting credit for previous experience.
JJ