As an Aussie it’s definitely the T’s that always stand out for me when listening to American English. Specifically the word “important”, to me it always sounds like it’s pronounced by Americans like “impor-en’”
When I was a kid there were some Australians in town for a youth lacrosse tournament just outside Philadelphia. The most confounding thing for them was figuring out what we meant by "wooder" (a popular beverage, sometimes fatal).
Many British English accents don't like the letter T. Eg Cockney might replace it with a glottal stop, turning butter into bu'er.
General American (or at least my version of it) replaces the t in butter with an alveolar tap.
Possibly shaped by the fact that people who speak New York accents reduce T's even more than most Americans. I'll tap, but most NY/New Jersey accents barely even glottal stop.