Other issues such as immigration process is a major cause of discontent at US aiirports when compared with other countries immigration workflow, particularly for non-locals.
One serious issue in Heathrow is that gate numbers are not shown (not even printed on your boarding pass) until 30 minutes before departure, and the gates are supposed to close 15 minutes before departure, but it takes 20 minutes to walk to some of the gates from the waiting area. Of course they bend the rules so that people make it to their flights, but what happens when they don't?