There are two reasons why I don't use GNU/Linux: One is GNU, the other is Linux. Of these, the larger reason is GNU, and specifically the glibc part. The most recent reinforcement of this is Ulrich Drepper's inability to read the C specification.
For those not familiar with the C specification, all identifiers that start with an underscore are reserved for the implementation (see section 18.104.22.168.2). You should never use them in your own code, because your compiler is completely free to do whatever it wants with them. By convention, single underscores are used for global non-standard libc extensions and double underscores are used for compiler builtins.
You can find a number of these in existing compiler. Microsoft exposes SEH with keywords like __try. GCC provides __asm for inline assembly, ICC uses __cpuid for accessing the CPUID instruction, and so on. Clang added __block as a type specifier for their variables that are copied to the heap for use by blocks (closures).
Unfortunately, it turns out that the glibc headers use __block as a parameter name. There are several things wrong with this. One is that they use double underscores at all. By convention, these are reserved for the compiler, while single underscores are reserved for the libc. The second is that they used underscores at all in a parameter. Parameter names are not in the global scope, so they can be anything to prevent name clashes.
The result of this is that, if you use glibc, you can't also use blocks. This is a shame, because we (Etoile) were shipping a working blocks implementation six months before Apple. Well, working on *BSD and Solaris (and probably Windows, QNX and Symbian with PIPS, but not tested there). This problem means that it doesn't work on GNU/Linux.
No problem for me. I only use platforms with libc implementations written by people who can read specs. It may be a problem for some of you, if you use a broken platform with a libc maintained by someone who'd rather salvage his ego than fix a problem, and if it is then I'm sorry for you. My suggestion is that you remember that there are other options.