Most portability issues that generate warnings are because we are not careful about our types. Standard library and arrays are indexed with size_t. Standard container sizes are reported in size_t. If you get the handling of size_t wrong, you can create subtle lurking 64-bit issues that arise only after you start to overflow the indexing of 32-bit integers. char vs unsigned char.
http://www.viva64.com/en/a/0010/
Most of the other concerns in this document ultimately come back to portability issues. Avoid statics is particularly of note.