1. That whole "u" thing
Reading "color" versus "colour" might clue one in to whether or not the literature in question hails from the United States or one of the Commonwealth nations. The latter favors the original "u" in words like the aforementioned and "neighbour" and "flavour" and the like.
2. Oxford comma
One of the most blood-boilingly controversial grammatical phenomena in the English language, the Oxford (or serial, or Harvard) comma — which separates listings of three or more (in "John, Paul, George, and Ringo," for example, it nestles itself behind Harrison) — rarely pops up in British English. American English, save for journalistic works, loves it.
3. Punctuation's relation to quotation marks
When it comes to quotes, Americans usually place their punctuation marks inside before moving on to the next sentence. The exact opposite holds true for British English speakers and writers, as they prefer leaving them on the outside.
4. Verb forms for collective nouns
Collective nouns understandably baffle English speakers on all sides of all ponds, but there's really just one general rule to keep in mind. While in (or writing for) Commonwealth nations, collective nouns — which include nation names — pair up with plural verbs. In the United States, use a singular conjugation.
5. Periods after titles
American English majors swoon over Mr. Darcy. British English majors swoon over Mr Darcy. Non-English majors have taste.
6. Placement of the day in dates
See, British people write out their dates like this: "13 January 2012," "13/01/12," or "13.01.12." While American people write out their dates like this: "January 13, 2012," "01/13/12," or "01.13.12." WACKY!
7. -ize vs -ise
Words that typically end in –ize in the United States and Canada are frequently rendered with –ise in every other English-speaking nation. However, because language wouldn't be language without numerous exceptions, sometimes the non-Canadian British English speakers rock that -ize as well.
8. Quotation marks
Single quotation marks are most common in British English nations, though their double counterpart has started creeping into daily use as of late. By contrast, Americans default to double quotations, using the singular ones to denote quotes within quotes.
Obviously, different accents mean words take on completely different pronunciations depending on their speaker's country of origin. Uh-loo-mu-num in American English is ahl-oo-men-ee-um elsewhere, most infamously.
10. "And" between numerical units
British English speaks or writes out numbers including an "and" in pretty much everything past 100, barring its multiples. "2012," for example, would be written out as "two thousand and twelve," while Americans expunge the "and" altogether and prefer "two thousand twelve" or "twenty-twelve."
11. "Through" vs. "to"
Exceptions, of course, exist when using "through" or "to" when denoting time, but — as per usual — one can stick to generalities here. In America, events run from "January through February," but British English typically prefers "January to February."
12. "With"'s adverb status
In the UK (and other nations sticking to British English), "with" exists exclusively as a preposition, but Americans adverb it up in phrases like, "I'm going out for a few beers. You want to come with?"
13. Compound nouns
Compound nouns consisting of a verb and a noun are typically shortened to bare infinitives in American English, but British English prefers keeping the –ing form (gerund) intact. For example, few Americans would don a swimming suit, but they wouldn't seem out of place in Britain — where, in turn, it might prove difficult to find a swimsuit.
14. "Zee" vs. "zed"
If the last letter of the alphabet is "zed," you might be talking to a speaker of British English. Americans prefer a nice, hearty "ZEEE!!"
15. Inflectional suffixes
American English loves them; its Commonwealth equivalent not so much. Hence, cookbooks would call for skim milk in the former, while cookery books want you to buy skimmed milk in the latter.
16. "Shall" vs. "will"
Both modal verbs, both popular in different English-speaking nations. British parlance typically sticks with "shall," while Americans tend toward "will."
17. Irregular verbs
Oh, geez. All sorts of differences exist in American and British irregular verbs, with the most notable involving –ed and –t suffixes. English speakers in the United States rarely, if ever, conjugate past participles as anything BUT –ed, whereas their counterparts swing both ways.
18. What, exactly, a "fag" is
Seeing as how one English form uses this word for purely innocent purposes and the other uses it as a nasty, hateful slur, it's integral to understand the distinction. In Britain, a "fag" refers to a cigarette, and feeling "fagged out" stands in for exhaustion. Once one gets to the United States, its meaning changes to serve as a pejorative toward homosexual men, so avoid using it at all costs.
Americans use quotation marks for reasons other than pulling from other sources — they're the primary advocates of sarcasti-quotes, which place the venerable punctuation in an ironic setting. Basically, air quotes on a page. The British rarely, if ever, do such things.
20. 24-hour time
Like the metric system, a good chunk of the world relays time in 24 hours rather than two blocks of 12. One can see both rendered in British English, but citizens of the United States — save for those in the military — exclusively write out the current time as the latter.