For balance's sake I will provide a (rather opinionated) description that is more favourable to the opinionated approach (in contrast to some of the other answers).
Opinionated frameworks provide a "golden path", which is supposed to be the best practice for most people and most scenarios (in the eyes of the authors).
This however doesn't necessarily mean lock-in. It means that it may require some extra effort to do things differently.
Less opinionated frameworks provide a number of different options and leave it up to you to decide.
Opinionated frameworks usually remove the burden from developer to reinvent the wheel or rethink the same problem again and again and thus help focus on the real problem at hand.
In the open-source world you can find many opinionated yet competing frameworks, so you still have a choice. You just have to choose your own golden path.
