The choice of a dissertation subject is very important, and it should be started long before you begin writing your paper. You can be a skillful researcher and a brilliant writer, but if you fail to make an adequate choice among lots of possible dissertation subjects, your work will probably not be a success.
Choosing your dissertation subject, consider its scope. Your subject should be quite broad to allow you to gather enough evidence and consider all the questions within your topic. For example, if you are assigned to write a dissertation in geography, it is not reasonable to write about the problem of pollution in the UK, because you will not be able to cover this topic within the scope of your dissertation. That means your subject should be quite narrow for you to be able to make a deep analysis of the problem. What is more, your investigation should fit in your word limit.
Dissertation subjects should be evaluated according to the student’s background knowledge. It is not a good idea to start writing about something which is absolutely new to you, you will not be able to develop it within the time you have. Actually, you do not have enough time to waste it on experiments.
You know that good dissertation subjects should be new. Still, it does not mean they should be completely new, you are supposed to investigate subjects that already exist. But it is important that you investigate this subject in a new, original way. Choosing a dissertation subject is to some extent more difficult than writing a dissertation itself. You will have to read a lot, considering possible dissertation subjects, to determine what has been done before. The results of your work will be very useful when you pass over to writing a literature review and proposal.
Make use of the resources your advisor can offer you, he can show you the way where you can find a subject worthy of research. Your supervisor will probably offer you some dissertation subjects to choose from, and it will be reasonable to accept this help. Choose the one appealing to you. Remember, that if you have support of your supervisor, your committee will probably be more loyal to you, too.
Finally, the most important aspect of a dissertation subject is that it should be interesting for you, as a good topic is the one that you like!