The biggest things I hear from IFR pilots from the USA are
- the impenetrable Eurocontrol route validation system (which has basically put $ millions into the pockets of Jepp and other flight service providers, without achieving anything else because ATC do not operate it on the flight)
- the airport restrictions (opening hours, PPR, PNR Customs, etc)
- the often massive airport charges, with $800 being quite common with the big ones
Americans do face some of "our" issues on the very rare occassions they leave the US mainland. On US pilot forums they moan about hassle when flying to Canada, the Bahamas, etc. I think some 99% of US piston pilots don't ever leave US airspace.