What is the difference between Oracle Spatial, PostGIS, SQLServer2008, SpatiaLite, and MySQL in terms of their spatial support. Are there specific use cases where each shines. What might you want to avoid with any of these databases.
Database Comparison Matrix
In this link you can find the matrix of comparison between databases with spatial capabilities.
It is a very broad question.
The bottom line is you get what you pay for.
Oracle Spatial which can only be used with Oracle Enterprise Edition. One can use Oracle Locator, but has less of the native tools for GIS. Spatial also has GeoRaster which is an image format stored inside the database.
The next level is SQL Server 2008, but they are very new to the Spatial Storage market. Most GIS vendors had to build application functionality to service geospatial information in this DB. But now with spatial capabilities, of which I am not familiar with this.
Now you get into the free world. PostGIS, SpatialLite, MySQL
PostGIS is the oldest open source, followed by the others. PostGIS is by far the most mature open source implementation out there. It seems that there may be limitations to these DB, but for a light implementation, possibly limited in functionality, they are sufficient for the masses.
One thing about PostgreSQL/PostGIS is that it is the most likely candidate for having a native interface present in other open source projects. For example, in QGIS 1.4.0, there is a button sitting right in the menu bar that says "Add PostGIS Layer". Spatialite also has a toolbar entry, but there is no similar support for other databases like MySQL.