Jocular answer:
When I was spending three years in the wilds of Orange County, US, I was hankering for the culinary variety of Old Melbourne Town. I kept telling anyone who would listen (and that wasn’t many) about how when I’d go back, eat at my favourite Malay-Chinese restaurant on Lygon St, and swim on a sea of latte.
When I got back after three years, I headed straight to the now sadly defunct Nyonya’s, had a satifsying noodle dish, and topped it off with my then favourite dessert of lychees and vanilla ice cream.
My biggest surprise when I came back: how god-awful Australian ice cream is.
(2001, it’s gotten better since.)
Somewhat more serious answer:
After six months in Greece: how standoffish and undemonstrative Australians are. I was actually surprised my thesis supervisor didn’t hug me to welcome me back.
More generally: reverse culture shock. It makes sense that the foreign culture is exotic. It doesn’t make sense when you’re alienated from your home culture.