What is non-trademarkable?

Several types of terms are not protected under copyright law and can not be registered:

marks confusingly similar to existing marks - A mark is not subject to protection if it is too similar in spelling, sound or appearance and is used for similar goods or services (a new sportswear company could not call itself Nikee).
generic terms - A mark can not be protected if it is simply the name of the goods themselves (a soft drink called Pop).
weak marks that have not achieved secondary meaning - The owner must demonstrate that the public recognizes the mark as unique to their product (Joe's Bar would not qualify).
abandoned trademarks - If the mark is no longer in use and the owner has indicated intent not to resume use, it is not protected.
functional features - A functional element of a product or its packaging, as opposed to a decorative element. Note that this may be protectable under a design patent.
titles of literary or artistic works - Titles of books, movies and music can not generally be trademarked (or copyrighted). It can be if it is used to brand a series of books (Chicken Soup for the Soul), or related products (Star Wars toys).