MySQL is a "lower featured" package than Microsoft SQL Server, but that isn't always a bad thing. Particularly if you use (or write) code that targets MySQL, it works just fine. The SQL sub-set that MySQL 4.x delivers is more than adequate for most purposes even though MySQL still falls well short of the SQL-92 standard that is the benchmark for the "big name" SQL servers.
MySQL is small, fast, relatively simple, and good enough for most purposes. MySQL is also cheap compared to DB2, Microsoft SQL, Oracle, and most versions of Sybase.
If I had to implement a system that required 24 by 7 runtimes, with tech support always on call, I probably wouldn't pick MySQL for the database. If I had to implement an application for a non-profit that could cope with occaisional downtime and placed a high value on fast response times on reasonably priced hardware, MySQL would be high on my list of candidate databases. You need to consider how the system will be used to be able to place a value on the featues that MySQL lacks, but in 90% or more of the systems used by businesses it is certainly "good enough" to do the job.
-PatP