Opinionated software is built and designed in such a way that it makes it easy to do things in a certain way. It favors certain design patterns more than others. In the process it makes it difficult to deviate from the style of software development for which it was developed. Another way of putting it is that it favors "Convention over configuration". i.e. The configuration options are very limited as the software assumes many of the configuration aspects. Opinionated software usually is quicker to master once the assumptions are understood.
Unopinionated software on the other hand makes few assumptions. And as a result, softwares/software development frameworks that are unopinionated often tend to have a lot of configuration options. A developer typically has to make a lot of decisions regarding various aspects of the software. Often, various tools are developed so as to make dealing with these enormous options easier. e.g. Visual Studio .NET for .NET, Eclipse IDE for Java etc. Unopinionated software typically takes longer to master than opinionated software.
