Be warned that quite a few countries will charge visa-on-arrival (i.e. US$25 for Indonesia at the time of this writing) and also charge a departure tax (i.e. 150,000rp for Indonesia at the time of this writing).
Also be prepared to have an alternate means of payment as some budget airlines, such as Air Asia, sometimes do not accept US credit cards online.
Dress ranges from conservative to liberal, so it's good to bring cool clothing for more modern, hot climates and longer sleeves and pants for religious temples and more conservative situations.
Harder to reach rural cities and islands may not have modern conveniences. Be prepared to say bye-bye to the internet, hot water, and flushing toilets in exchange for some amazing experiences.
Citizens of Malaysia and Singapore tend to speak fluent English, but this decreases as you venture to more rural areas. Japan can be difficult because everything is written in kanji and Chinese characters, so it may be a good idea to have an English copy of the subway system. China with its multiple dialects can be challenging, but the signage in the subway systems is written in both Chinese and English.