I created new databases and tables to have a fair comparison for performance testing. There were no indexes at all used for testing. That wasn't meant to be a robust test of the capabilities, but more of a validation of what was happening and why. I agree that with the right architecture and strategy it's entirely possible to make MySQL perform well, I was just surprised to see such a dramatic performance difference in some relatively simple queries. It's always going to be necessary to do some work on performance tuning to get the best results, I was just surprised with how much earlier in the process that's required under MySQL.
I keep coming back to trying MySQL and finding it disappointing, usually related to work. I've been using PostgreSQL since 1997 and it's always worked well for me. The most interesting comparison isn't between MySQL/PostgreSQL, since that is a religious war with a lot more people on the MySQL side. The best comparison is how easy/difficult it is to migrate from one DB to another. Migrating from PostgreSQL to Oracle has been very simple when I've done it. Everything just basically works the same out of the box. If it's common to need to do so much MySQL-specific performance tuning, I would see converting to/from it to be a much more difficult project.
I really wonder how much time and effort goes into large MySQL databases and how much is related to how it works. Basically, from a practical standpoint, is that money being spent on people's time costing more than Oracle licenses would? I'm not even a big Oracle fan, but I have developed data warehouses on Oracle and I can't argue with using a tool that works. At the end of the day, finding the right solution to the problem is what counts. It would just be interesting to see cases (other than small, simple databases) where MySQL really shines. A lot of people really like MySQL and use it for some pretty large and business critical applications. I would like to think there's more than ignorance of other databases that drive those decisions. =)