Oracle - For Oracle we talked about how Oracle grew with an incredible sales team and as Oracle grew, the feature/functionality grew with it. Now it is the aircraft carrier of database servers. If I'm a Fortune 1000 company and I need to run financials, Oracle's Jedi skills are strong in this area. I've worked with Oracle technology for 20 years and I am a fan of Oracle for the right environments where cost is not an issue. If you are in large complex environments, Oracle has strong complex tools for solving complex problems.
SQL Server - Great marketing. Whoever came up with the name of "SQL Server", this name is brilliant, this person should be drinking pina coladas in Aruba. Great point and click database functionality with good price points compared to Oracle and DB2. In a world of point and click people, SQL Server is popular and easy to work with relatively speaking.
MySQL - An incredible wave of customers looking at the great price point of running MySQL servers for projects. The cost savings for projects can be incredible. MySQL is like the friendly kid on the block that has cool toys. MySQL makes sense to be able to run in environments that are also running Oracle, SQL Server and DB2 for a great price point on separate projects. So MySQL plays nice with everyone. It also has great features like replication and clustering. MySQL also has great portability with Linux, Windows, Solaris, HP-UX, AIX, etc. MySQL also has very simple upgrades compared to other database servers. The power and popularity of open source is growing dramatically. The Jedi powers of open source are very strong. As Linux has become incredibly popular as a low cost powerful solution, MySQL has become very popular as well. A lot of companies are realizing on a lot of projects they just need core relational database ACID features and in this environment companies are seeing MySQL as an excellent solution.
DB2 - Someone described DB2 by comparing it to the T-34 Russian tank used in WWII. Not pretty, fancy or a lot of features compared to products like Oracle but it does an excellent job as a powerful database server.
Anyway, just a few shared conversation thoughts on different database servers all tongue in cheek. :)