Main language spoken
If your home language is English (or you can speak English very well, as most South Africans can!) then you can visit countries and regions where English is commonly understood, such as the United Kingdom, United States of America, Canada, Botswana, Swaziland, Namibia, Zimbabwe, Malawi, Zambia, Lesotho, India, Singapore and the Caribbean islands.
If you are able to speak other languages this list can obviously be expanded.

Geographical factors

Consider what you want in terms of temperature, weather, and seasonal events (like hurricanes, tornadoes, floods, flowering of daisies in Namaqualand, whale watching)

Social events, holidays and religious periods
Travelling over high season is much more expensive than of you travel out of season.
Find out the public holidays and religious days and holidays – working weeks differ according to whether the country is Muslim, Christian or Jewish. Events (like South Africa hosting the football world cup in 2010) and festivals (e.g. the annual Rio carnival) could also affect your dates and costs of travel and accommodation.

Business & economic cycles
When do prices increase? Air tickets change usually before and after high season, and after large changes in the oil price (rarely down though!)

If you are planning to work
Different industries have different busy periods (e.g. November and December are always busy for retailers in Christian countries)

The timing of seasonal work, such as crop picking, depends on the crop and hemisphere.

The relative strength of your currency to that of the country you are going to work in (e.g. South Africans often work in the UK due to the strength of the British pound). if you are going to work for or with a government organisation the beginning of the financial year can be important as this is when the new budget starts

if you are planning to study
academic years start about August in the northern hemisphere, and in January in the southern hemisphere

Cultures and traditions
Many countries have restrictive or conservative dress codes, hours of work vary, with many countries closing down every day for siesta. Labour laws can restrict the number of hours you can work (or that the employer pays normal time for)

Legal and administrative systems
Some countries insist on visas and/or work permits (depending on the nationality of your passport) that restrict your options or length of stay. Many developing countries have restrictions on the amount of foreign currency you may take out. Liquor and medicines can be difficult to obtain in many countries