While US spelling is the most immediately notable difference twixt the two forms of English, there are far deeper differences. US English has only three tenses (which is why US speakers say things like "I didn't do it yet") and mostly relies on two verbs ("I got laid last night"). There are no adverbs. Meanwhile the vocabulary is likewise stripped down, with the average US speaker using 500 words per day while their British counterpart employs 3,700. US English has evolved to be quick to learn, which is ideal for a land of immigrants hailing from a great diversity of languages. It can't afford the weight of British English, which relies on the fact that until very recently most speakers were native born and thus acquired its complexities over many years of childhood.