Vocabulary, spelling and accent, obvious. For example, the word "dumb" has different meanings in UK and US (look out !)
Several verbs have different simple past or past participles,
Such as in UK, I get got got, in US I get got gotten. In UK, to bet I bet bet, in US to bet I betted betted, etc.
Important : the perfect tenses are used far less in US than in UK . So the grammatical rules of these times use are often not respected.
What's more "I have got something" is much more used in UK to mean "I have something"