This site also provides a bunch of sites with examples of what you can do with DNN, in the menu at the top, under Community, Showcase Sites.

Regarding developing custom functionality for DNN, in the form of custom modules, you have to ask yourself how accomplished you are with ASP.Net development.  If you are experienced, then DNN will make your life easier since it will provide you with all the framework to manage the skin, authorization and authentication, caching, and many other things and you only have to write the functionality you need.  If you are not very experienced in ASP.Net to start with, developing for DNN will be a little more complex than straight ASP.Net.  But then, you have to ask yourself other questions, do you want to write, test, and roll-out your own user login, authorization, and authentication scheme including accounts management (this is very robust in DNN), as part of security, do you want to write your own security access management via groups/roles?  Again, this is very strong in DNN.

DNN is not for everybody, but if you are committed to ASP.Net then you should give it serious consideration.

Carlos