Poco is a great library and you can do every thing which you need in network based applications; However, it is suffering from lack of containers and some basic class such as string. In my point of view, adding thread safe containers such as map, vector and ... is essential. because, you need to use additional libraries such as Boost or libraries such as STL which aren't thread safe.
Handling different code-pages or Unicode formats in an HTTPServer is not an easy job, and programmers have to struggle to handle inputs. creating a abstract string class which have ability to deal with different code pages and Unicode string automatically without interfere of programmer could be useful.
I suppose, adding a few range of classes would make it (Poco) a great framework for developing software. It might be able to compete with other powerful libraries such as Qt.