Your question is quite broad (and hence difficult to answer) but I will try to address the point of why someone might raise this as a criticism, which I guess is part of what you are asking.
When Rails first came out a couple of things about it were:
- It was a full stack framework (ORM, MVC framework, template engine, collection of helper methods which all worked well together) - this removed the need for the developer to choose a package in each of these categories and to get them to work together.
- It was opinionated software which again meant that a lot of decisions were made for you, ranging from the folder layout for your project to the default names for database tables and fields when using ActiveRecord. Again, this helped free the developer from making these choices and let him or her concentrate on the specifics of the project at hand.
Much of this still stands but there are certainly more potential choices now than in the past. e.g. if you prefer a different template engine, don't need an ORM for your project and so on. This is generally a good thing as mentioned in the post that tokland linked to.
You mentioned that you don't know much about Rails. If it's something you're interested in learning more about then I would start a project with the default choices and see how that works for you. Then you could post follow up questions like "Can someone recommend an alternative template engine I could use - I don't like erb because of specific reason XYZ"