One of the biggest differences for me was the US banking system and general money culture here. Parts of the US banking system seem so behind the times. For example, you cannot just freely send money from your bank account to anyone else's bank account at any other bank in the country. Cheapest way is to write a check.A few banks, like Chase, do have ways to send small amounts of money to other Chase account holders though. Lack of two-factor auth with some bank's online systems is also particularly concerning. Landlords still want to be paid with checks (as they're free to cash), and the concept of wiring someone money to pay for stuff is totally foreign to most people I meet. Thankfully, a lot of employers seem to have finally caught onto giro credit (known as 'direct deposit' in the US), meaning you can get your salary paid directly into a bank account (or multiple, if you'd like) and not have to have physical checks sent to your home address.
This was something else I was also going to mention. Anything to do with banking/financing/insurance is generally archaic in the US compared to UK. One of the projects I'm working on at work is to help them achieve a similar system in the US to what we have in UK when taking out motor insurance online. They still have broker offices etc in US you have to physically go to in order to get insurance certificates. It's amazingly old fashioned really.