I like its popularity and the way it is easy to find information about its usage and configuration.
Almost every framework or library comes with instructions about how to install it on Apache, and most part of web applications run Apache
At time, it happens that some features, like the redirect module, can be very difficult to understand and configure, even with the many instructions you can find online.
However, to really master Apache you need good training, hopefully with dedicated manuals and guides.
Still, Apache has serious performance issues with specific configurations, and unresolved serious troubles, like the php fastcgi modules breaking chunk uploads