Menu flow, scene manager, path finder concepts, difference between static game data and dynamic game data and how to connect them (e.g. RPG::Actor and Game_Actor), clever generalization and usage of object oriented design, model-view-controller concepts and possibly other design patterns.
EDIT: How to store data (e.g. RPG::Actor), i.e. what are the advantages and disadvantages between different formats (e.g. XML is human readable, can be edited with Notepad and needs more programming code to be read properly while some binary formats are technically encrypted, require an additional program to be edited and they require less code to be read) and when SQL databases make sense.