They are 3cm taller
10lbs heavier (this disparity increases significantly in support arms and after retirement)
Shoes are two sizes smaller - (its true what they say about shoe size)
Tolerance for alcohol is 83% lower
My BIL is a retired US infantry officer. Obviously I've never served under him, and nor would I on general principle
He is however a sound guy, and well-grounded. US officers come in three main types; those who went through the academies (West Point, Annapolis and Colorado Springs), those who went through the ROTC or those who did OCS (commissioned ex-rankers). My BIL is of the ROTC route, but was an instructor at West Point for 3 years. We visited him there, and the college I thought much like RMAS.
The program is much longer, a 4-year degree course, but the end result seems comparable to me. The ROTC and OCS guys won't have the same career prospects in the main, but they're not as stuck-up as some of the USMA guys can be. When I lived in the DC area and was on the Embassy circuit, some of the academy-route officers I met there could be a bit stuck-up.
My BIL does at least have a sense of humour. We were working on our (my and his sister's) land a couple years ago, cutting trees down etc. He's sweating his tits off. I'm taking a break. "You just drink your coffee sergeant". "Fcuk off colonel, I'm ball-bagged" We just fell about laughing.
Worked with a fair few for the past 5 years in Afghanistan and on the whole they are pretty good. What surprised me was they have a lot of National Guard deployed and not that the subject came up initially but after few weeks of finding out a person was reserve where there is a marked difference between our Regular and Reserve offices in quality, it seemed less so in the US mil.
Having trained and worked alongside them, I’d say the key differences are probably...
(Warning: Comments below contain massive and probably unfair generalisations...!)
Professional Education - The US Army (and not just Officers) take it much more seriously than we do. For them, attending career courses is actually important, they gain Masters level qualifications for attendance. Whereas your typical British Officer Education course, certainly up to ICSC(L), is barely an attendance course. There’s no point in trying, the correct Regiments will make up the top third and it’s not like the mil qualification is worth anything in the real world, is it?
Which leads me onto ... attitude. From what I saw, US Army Officers adopt and maintain an incredibly professional attitude, although I recognise that might be a front they put on in the presence of non US personnel. This sadly translates into a culture of process over product where a tent full of US Army Officers will diligently slave away over an insanely complex slide deck. It won’t matter if the deck is utter pump, what will matter is the followed the correct decision making process to get to the pump result.
The British Officer by contrast is probably much more happy being an enthusiastic amateur - a product of our Generalist Staff Officer model I suspect - but is probably a lot more comfortable to ditch process to get the right outcome. If the obvious answer is screaming in their faces, the process gets rapidly ditched in favour of the result.
I know it sounds a bit daft, but the most obvious example of this is of course dress. Finding two US Officers dressed identically is easy. Finding two British Officers dressed identically is bloody impossible!