Regarding the strong fundamentals: - A clever, extremely flexible, roles/permissions system. It is based on giving CRUD permissions to user roles, allowing to define access rules based on the fields (for instance a READ access with a record rule [('user_id', '=', user.id)] would allow to read only own records). In most of the software permissions are expressed in the code, in Odoo they are abstracted and enforced at ORM level. - The custom ORM system is very strong and allows to create objects and fields at runtime. You can create a model and its fields and start using it right away without restarting the server.
Some of the smaller bright ideas: - Records navigation is rather smart: the pagination system allows to manually define the records range to be seen avoiding the usual "records per page" dropdown; standard filters can be defined using the same domain syntax from access rules above (the filter for "My records" would be [('user_id', '=', user.id)]). - Many views use kanban, I find them extremely practical to get a good overview, in particular for CRM opportunities and candidates screening processes.