PostgreSQL, I wish

Some of that is happening (wal_buffers is autotuned since 9.1, max_fsm_* doesn't exist anymore since 8.4, default_statistics_target has a sane default since 8.4, 9.3 won't need adjustment of sysv /proc settings for most things), in most of the other situations its either hard to do or it very much depends on the environment. Many pg instances don't run on a dedicated machine. Many database servers serve multiple pg clusters at the same time. Many database servers run in virtualized environments where you get better average performance if you use less memory.

There are other variables where nobody fought enough to get them changed. Other changes are beneficial in a wide range of systems but hurt others badly...