For some reason, the set directive is available only within server block. Wouldn't it be possible to handle set's also on the top level?

Motivation? Well, some use case:

set logdir /my/log/dir; error_log $log_dir/error.log

# ... http { # ... server { access_log $log_dir/access.log combined;

Similarly one could set crucial ports, hostnames etc once and reuse them thorough the file.

Another possibility - one could write whole server config using variables for directories, ports etc, then include it into files which would just set those variables differently (and - say - reuse the same config on different machines).