A major issue for European pilots is that the majority does not have English as their native language.
American English, especially with strong accents, can be difficult to understand over bad VHF because it differs a lot in some regards to what most Europeans learned in school.
Likewise, Europeans will always have some form of accents either weak or strong. I feel it is up to the Americans to accommodate this when European traffic enters the US. Lead by example.
In reverse, I find it highly annoying that Eurocontrol still allows non-English communications. While most Europeans will get the hang of this, for American pilots it must be terrible.
In general, I am of the opinion that the "hosting" party should do their best to accommodate the "visiting" traffic.