As an expat Brit who has lived in the US for 16 years it's the little differences that take some getting used to.
- Getting a social security number
- Obtaining a credit card with zero credit history
- Renting a place to live with no credit history
- Buying or renting a car with no credit history
(you can see credit history is a theme here!)
- Working out how much tax to withhold
- Filing your own taxes every year
More obscure, but noticeable things for me included losing cultural references for while. From sports to TV etc. there will be many references that pass you by and many references you make that nobody will pick up on. Of course that will be addressed in time.
On a lighter note, I worked with an Australian in NYC one winter. After a bad storm, the next morning he was asking myself and other colleagues "Is there a trick to walking in snow? I left my apartment last night and fell over 3 times!". Obviously not such a big deal in California.
Having to pass a new driving test was a hassle, but not a big deal. Of course, you will be driving on a different side of the road, but that's easy enough to pick up.
Probably not quite the same in California, but in the North East, when I ate out, the portion sizes were huge. Very easy to put on weight. The food seems to contain more sugar, so be wary of that too.
Alcohol laws are different and vary from state to state. In some, if you serve an 18 year old a beer in your house, you can go to jail.
I never viewed any of these as negatives, but they all add up in the first few months.