[brudgers]

Why? Not being facetious. Wondering about the weights of benefits versus costs for users.

[codr7]

Mostly portable implementations of extensions for me; __thread, bool, inttypes.h, variadic macros, flexible array members etc. But declaring variables closer to the point of use is nice, as is struct initializers, compound literals and single line comments.

Compared to the rest, C99 contains a lot of useful improvements.

[kick]

The only real cost is that Windows users will have a slightly less convenient time, which doesn't matter for most projects.