I've finally decided to learn everything I could about MySQL performance tuning; we're working on highly tuned appliances and this is a must for high-traffic environments.
I'd like to share my first findings on interesting stuff and encourage comments on the matter, which seems as deep as any science.

These last days we've discussing about this at the office and we couldn't agree about the type of database configuration using MySQL was optimal for the broadest range of users.
It's much easier to tune everything if you already know the exact environment, available hardware and so on, rather than trying to tune a database for a broad range of people going to install a product.