As an Australian who reads a lot from both UK and US sources, I can say the differences are (in respect of the whole language) fairly small, but they can trip you up; not so much spelling (ours is mostly UK but still a mishmash) as you learn quickly to skim over what is 'right' in one or the other style. But actual word usage can be a bear (think of rubber, pants, jumper, trunk, flashlight... etc etc etc). And of course the problem is you don't KNOW you've got it wrong until someone TELLS you....