Being unaware of heaps of existing and extensible functionality in the framework. Things often redone are membership, roles, authorization, site maps. Then there are the controls and the associated tags which can be customized to alleviate issues with the client IDs among others. Also simple things like not knowing to properly use the .config file to auto import namespaces into templates, and being able to do that on a directory basis. Less known things like tag expressions can be valuable at times as well. Surely, as with all frameworks, there is a learning curve and always something left to be desired, however more often than not it is better to customize and extend an existing framework instead of rolling your own.
