I know that all variants of English (American English, British English, etc.) can be generally understood by everybody who knows any of the English variants. However, there are some regionalisms that can lead to ambiguity.
What are the things I must be aware of when the target audience (of my writing: book, blog, etc.) is the world?
If your target audience is the world, then you target not only people with a knowledge of American English, British English, and so on, but also people like myself, to whom English is not a mother tongue but a foreign language. If reaching these people is important to you, then you might want to write in a relatively simple English, avoiding not only regionalisms, but also:
Overall, providing a lot of context, and writing simple, concise sentences will help make your point clearer (and not only to “foreigners”).
I would suggest writing in the version of English you are most comfortable with. That's what you'll communicate in most clearly, and everyone else will understand it from that domain.
English is broad church (British English bias intended :).
My preference in many circumstances would be to not unnecessarily burden yourself with removing any regional colour from your English writing. Seeking to do so will eventually only make English into a more generic and less expressive language. If the writing is online you can always make a helpful hyperlink or two.
It's important to be easily understood, but that's not the only important thing.
The answer to this is included in Postel's Law: “be conservative in what you do, be liberal in what you accept from others”.
My suggestion: go with what you are comfortable with. For example, I'm an avid blogger, and I like writing, and I use British English (with a touch of our own Caribbean slang and other things) as my style/mood requires.
You need to be more concerned about using correct grammar and universally correct spelling (there are some differences between American and British English, but those are not universal spelling/grammatical errors/issues), than about whether you use American/British/Canadian/Martian English :).
In fact, you could more tend to find yourself in trouble should you try to adapt to the variant of English that you are not accustomed to.
I once researched how the English Wikipedia handles this, because I figured it must be a real issue for them, too. (On a much larger scale than for you — no offense. :))
It seems like they are pretty laissez-faire on it. Basically, their Manual of Style says:
[...] the English Wikipedia prefers no major national variety of the language.
They're just trying to stay consistent inside an article, and to focus on one national variety when there are 'Strong national ties to a topic'.
Not sure how much this is really helping you, but I thought it was an interesting (and for me surprising) approach. I expected them to focus on one variety, but I think I started my research when I realized that I had just read two article using different spelling varieties.
Use American English, I mean general American English. There are too many varieties of accents, and vocabulary, in England, so much so that people learning English around the world wouldn't understand most of them. Cockney, Georgie, Scottish, Irish, etc. Those accents are not popular.
The USA has spread its accent through movies, TV shows, lectures, etc., so all over the world people understand it, but people do not understand most British accents. I should know, I do movie subtitles, we have to pay extra if a movie has no script and it's not American English. We may pay double or triple the amount if the movie is British because it is hard to find someone who can understand the accent(s).
To sum it up, go with what most of the world are used to. Of course, this is the opinion of someone who has to hire translators, so I can say that the only British variety that is cheaper to translate is RP, as it is taught everywhere, the other varieties are too expensive for my taste.
