QDataSets are less abstract and more flexible data model for das2. das2's old data model was developed to deliver spectrogram time series data sets where the dataset structure would change over time, and the interface is highly optimized for that environment. It's difficult to express many datasets in these terms, so the simpler "quick" QDataSet was introduced.
The QDataSet can be thought of as a fast java array that has name-value metadata attached to it. These arrays of data can have arbitrary rank, although currently the interface limits rank to 0,1,2,3, and 4. (Rank N is proposed but not developed.) Each dimension's length can vary, like Java arrays, and datasets where the dimensions do not vary in length are colloquially called "Qubes."
QDataSets can have other QDataSets as property values, for example the property QDataSet.DEPEND_0 indicates that the values are dependent parameters of the "tags" QDataSet found there. This how how we get to the same abstraction level of the legacy Das2 dataset.
This is inspired by the CDF data model and PaPCo's dataset model.