Drupal's current theme system that has still lots of concepts from ancient Drupal versions has shown us it might need some upgrades. Examples likeand shows us that the technical limitations of the current theme system, and forces us to make unpleasant decisions.
Biggest problems are:
- There is separation between admin and user interfaces even though they mix up
- Too many things work still by magic (hooks) and requires knowledge of how the system is supposed to be used
- Supporting BC for frontend is expensive
User interface changes
Data model changes