I found some vast differences between Australia and Japan when I came here on my working holiday.
First, it all seems very "clean". You won't see cigarette butts, food wrappers, or vandalised public property. The Japanese respect their environment so they look after it (in some nightlife areas you will see a few cigarettes on the road the morning after, but that's it).
Also, it's very safe. You can leave your bag on your seat and use the restroom in a diner, and it will still be there when you come back.
I do not know of a Japanese restaurant that accepts tips, so don't tip. They will get embarrassed and think you gave them the wrong amount of money. Also, most restaurants or bars will hand you a wet towel to wipe your hands with when you enter, some are wrapped in plastic and have been zapped in the microwave so they're nice and hot.
A safety note: I don't know about where you come from, but in Australia cars must stop at a pedestrian crossing if a person is crossing the road there. In Japan, cars are not obligated to stop even though it's a designated place for pedestrians to cross. So don't just walk out onto the road, be careful. Oh, and people run the red light. All the time.
Nightlife in Japan is nuts. Parties usually start at 10pm - 12am, and don't end until the early hours of the morning. So look forward to some crazy nights! Trains end quite early though like around midnight and start at 6am, so watch out for that (in Kyushu anyway not sure about Tokyo and other cities).
It's very popular in Japan to have "all you can drink/eat" sessions at restaurants and clubs, called nomihoudai for drinks or tabehoudai for food. They are usually limited to a couple of hours at most and it's a flat fee for all you can eat/drink (e.g. 3000 yen nomihoudai for 2 hours or something). Those are fun!
And one final note, you will probably get a lot of people who want to do language exchange, so be prepared.