Actually, Olaf, the worst things about MySQL, to me, are:
It scales poorly. Simple tests show a rather non-linear increase in response time as the number of writers increases. This means that if you have to wait 2 seconds for it to write a record for one person, instead of it taking 4 seconds to write the same amount of data for 2 people, it will take 6 or 8 seconds. God help you when you get to 20 or 30 writers. This problem exists with both myisam tables and innodb. It is the primary reason MySQL AB has NEVER published benchmarks that do things in parallel. It is the reason Postgresql is used by OSDL to tune the linux kernel, it does scale well.
It has poor SQL compliance. It has issues like using ` to quote identifiers instead of ", ignoring -- as a comment unless it has a space after it, allowing you to drop a table with dependent tables under it. It allows the following incorrect syntax:
select a,b from table group by a
and many more poorly thought out choices.
You can insert the number 4039487384765638465 into an int4 and you get 2147483647 as the number, no error, no fuss, no muss.
You can insert the number 1 into an enum, and it will take it, even if your enum was defined as being 'a','b','c', because 1 corresponds to one of those fields as an int.
The list of ways MySQL can screw up your data goes on and on.
Finally, MySQL GPLd the connect libs in 4.x, so now either all your code is GPL, or you buy a commercial license. They try to sugar coat this and say that if you're writing free software you shouldn't mind, and if you're writing commercial software you owe something back to the community, but what they're really doing is picking your license for you.
Wanna write a BSD package against MySQL and distribute it? you can't, unless you use the commercial version of MySQL.
The reason it's so widely available is that in the past, it was the easiest database to set up and administer in a shared environment (back when postgresql had VERY course security setup, and required a bit more hand holding / dba work than it does today.) Plus in the past the connect libs were LGPLd, so you didn't have to worry about the viral nature of the licensing biting you.
Just because it's the most popular doesn't make it the best, and vice versa. There's things I like MySQL for (mostly read content management systems are a good fit) but for serious database work where your data's integrity counts, look at the other two open source choices.
Speed is important, but it's not the most important thing. just ask the folks who sailed the Atlantic a long time ago in a fast ship called the Titanic.