Disable query cache by default

The only red flag that I saw at first glance was your query cache
usage. You mention using 20% of available RAM for it and I would very strongly recommend against doing
so. I would generally recommend that the query cache be disabled. 20% of total ram would
generally be bad. The reason for this is that the QC uses global write locks and it essentially
serializes access to mysqld in a single threaded fashion. It's very bad for high concurrency workloads. It
also holds a server level write lock when doing big purges. If your query cache is even 256M in size
then these purges can lock everything up for 10+ seconds. 20% of ram could easily cause very long
hangs.

I would also set the query_cache_type to OFF by default, as even with a
size of 0, it's locking is problematic in high concurrency environments.

We've done a lot of work recently for our defaults, as part of the 5.6
release. You can read about that work here:
  https://blogs.oracle.com/supportingmysql/entry/server_defaults_changes_in_mysql