Great thread. Only halfway through but wanted to add mine:
Moving to the UK from Johannesburg and discovering the miracle of Royal Mail 1st class stamps. You send a letter and it arrives somewhere else the next day
You may as well have told me it was delivered by the owls out of Harry Potter, I'd have believed it.
On a related note, the fact that my mail arrived to my flat, rather than my driving 20 minutes to a shopping mall to check my mailbox.
The size/cost of London housing. My first lodging was a single room at UCL, maybe 2m x 4m. It cost £100+ a week. I'd moved from my family home where I'd had my own large bedroom, bathroom, study and a swimming pool that I'd only had to share with the family dog.
The freedom to walk the streets in London, after getting used to driving anywhere. In the same vein, a broadly reliable public transport system - I could read books! While travelling!
Realising how much of a role class still plays in the UK, consciously or unconsciously.
The UK/London/some bits of London mentality of "Oxbridge uber alles". Still astonishes me.
From my DH: seeing 18 year olds in Israel (doing their army service) wandering around on public transport with large rifles.