Is using the same email and password fields to signup OR login for registration a good idea?

What would you like to accomplish? Get more users? Create signup or login process easier and faster? Or something else? When the user will come to the same form where he registered his account, won't be it confusing? I understand that one form can make this process simple and faster but i am afraid that not for users. Imagine that you would like to create new account and you have own expectation and needs. You expect clear names of forms and you are often looking for "Login/Sign in" or "Registration/Sign up". These names are really helpful for your orientation and expectation. In registration process, websites often need more information about users and these options are neccessary for different goals and business needs. But in login process, you don't need to fill these options and users would like to use only their credentials because they are used to use it in this way. In my opinion, one form for two different processes in this case is bad idea.

While the concept sounds good on paper, it is not. You said yourself that this approach is "opposed to the more traditional way" of having separate forms. Your users are going to expect your site to behave in the traditional way so your user experience should be designed that way. This rule applies for more than just the registration/login process too.

Amazon does something like this, it depends on what is being required by the system for the user to log in/sign up and the reason it is required. The system by Amazon requires email addresses because the email address is constantly used throughout the interaction with Amazon, for sales confirmation, reciepts e.t.c  If you then design a system which asks for usernames, a new user may be confused over this. I do think it can be implemented nicely but context and content are more important.

I came across the same issue some days ago when reviewing a client's checkout form. The first reaction that I had was negative, as I thought this solution goes against conventions and sticking to conventions is often the best choice when it comes to checkout forms. But, at a second thought I realized that asking for a single information, the email, generates so little content overload that users will probably insert their email without even notice that something is different.