I was thinking about what Niels said about the emphasis of tabs being reduced in the new release of Umbraco and how this could affect me as a developer. We currently build many bilingual websites which means rather than creating two versions of the website using host header mappings, we create additional tabs and add additional controls to each template which allows users to switch between languages easily.
This creates document types that can become quite overcrowded and the tab system has always been quite good for this.
If its good practice to reduce the number of tabs being implemented then it might be a good feature to have the ability to include groupings per document type. This would look similar to what HTML groups do on forms, with a legend, etc.
I think this way you will get things organised nicely together but on the same tab ultimately helping to reduce them.