I'm going to edit this post with updates whenever I think of a good new one.
English/American linguistic discrepancies not often brought up:
'Specials' In America this word covers a couple of different things. When applied to food, as in 'lunch special' it essentially means meal deal.
'Today's lunch special: one main dish, one side and one drink for $10'.
'Special: 2 slices and a can of soda $5'.
"Can I get the special?" "Yeah what kind of bread do you want?"
Specials can also refer to one-of a kind events or programmes. Common in TV.
'Tuesday at 9pm, an MTV unplugged special with Justin Timberlake'
Basically special is used commonly as a noun not just an adjective. We would usually use the word deal instead.
'Bless your/her/his/their little cotton socks' doesn't really exist in the US. I found that out today.
Sled = sledge
Sledding = sledging
When you say thank you, many many Americans reply with 'uh huh' 'mmhmm' or 'yup' instead of 'you're welcome' which I used to find mind-bogglingly rude...imagine my disappointment in myself when I realised I had started doing it. I'm paying close attention now and stick with a 'no problem' or 'sure' which are both more Americanese but don't make me feel so dismissive.
This used to especially bug me when I was working in the AMDA student store - I'd serve a student, give them their purchases, all with a smile and a hi how are you, then I would hand them their change and say thank you. You know like, thank you for your custom. And so many of the little bastards would go 'mmhmm' as they walked away. Wait..I just gave you courteous and friendly service you should be the one saying thank you anyway! Ungh.
Sneeze etiquette - I reflexively say 'bless you' when anyone near me sneezes. I'm used to hearing it from any random person nearby when I sneeze, sometimes more than one. Strangers, friends, whoever. Here, noone ever says it. It just isn't a custom, it seems. I still feel mildly affronted when I sneeze and don't hear the bless you I'm automatically expecting.
Articles of clothing
Trousers = Pants
Shirt = Button up/Oxford
Top (anything that is not a tshirt or shirt, can be prefixed by 'tank' 'long-sleeved' 'spaghetti strap' etc) = Shirt
Blouse = Blouse
Shorts = Shorts
T-shirt = T-shirt/Shirt
Polo shirt = whaaaa???
Skirt = Skirt
Jumper = Sweater
Jumpsuit = Jumper
(bottoms, tops or both) = Sweats
Trainers = Sneakers, Kicks, Tennis shoes
Converse = Chucks/Chuck Taylors
Tights = Tights, hose, stockings
Underwear (ladies) = Panties
Underwear (gents) = .....err...I think it's just underwear
Waistcoat = Vest
Vest = Undershirt
Confused? Find some of these funny in their pointlessness? Me too.