For the purposes of this answer, I will define the “Asia-Pacific Region” as the regions of East Asia, Southeast Asia, and what is often called Oceania. In other words, I will not include countries like Mongolia or Kazakhstan.
Even if we define the region in this way, it is not possible to identify important similarities that apply to all countries in the region. It is surely impossible to find much commonality between the Federated States of Micronesia, Japan, and Australia. Therefore, the similarities that we find will have to be limited to smaller groups of countries.
There are clearly similarities in culture between many of the countries of this region. In East and Southeast Asia, cultures have been strongly influenced by China, particularly in times long past. Chinese writing, for example, was used by people in Japan and Korea. Ethnic Chinese spread to live in many countries, including places like Malaysia, Vietnam, and the Philippines.
Much of the region is heavily influenced by the ocean. Japan was and is a country that depends on ocean resources. The Polynesian and Micronesians depended on the ocean almost completely. The weather patterns caused by the ocean (such as typhoons or the monsoon) affect many of the countries of the region.
In these ways, we can say that the region is one of similarities, but the similarities do not include all of the countries of the region.