As Rusty mentioned, for most airports in the world the transit is completely airside and no visa is required. The main exceptions to this that I know of are the USA and Russia (and the UK when transferring between LGW to LHR)
Transfers in Abu Dhabi, Dubai, Kuwait, Doha, Hong Kong, Singapore, Amsterdam, China, South Korea, Brunei and most other countries are completely airside and are fairly painless. No visas required.