Good annotation requires distinguishing between your reaction to a text and what it actually argues. This is one of the most important skills you need to get to do real scholarship. It makes your work both easier and more credible. Accurately summarizing an argument is an essential task not only for producing academic work, but for being a responsible, informed intellectual. It’s sorta like being a good listener who doesn’t constantly distort, judge, react to, or put words in the mouth of your friends in conversation. Saying what you like about a text, or how it will relate to your research paper, is not what an annotation is for. Being able to find some things that are wrong, contradictory, or offensive about a text is NOT the same thing as being able to summarize the argument.*
The annotations should contain information useful to anyone studying in the area (e.g., fellow students writing research papers). It should not summarize the article or book or list its contents; it should briefly explain 1) the author's main point or argument; 2) the author's method or approach; and 3) the place of the piece in the literature as a whole (e.g., does the author agree with other scholars?). The bibliography as a whole should provide a solid overview of the existing state of the discussion in the area you are studying.
Here's an example of a good annotation:
Fjellman, Stephen M. Vinyl Leaves: Walt Disney World and America. New York, Routledge. 1992: A detailed, largely anthropological analysis of Walt Disney World in Florida. Written with an obvious passion and love for Disney, Fjellman describes the corporate craft of Disney and how the political, ecological, technological, and entrepreneurial systems (marketing and promotion) have been developed and manipulated to create WDW, and the peculiar vision of history it promotes. He argues that Disney World acts as the muse for the allied transnational corporations that sponsor it as well as for the world of late capitalism, where the commodity form has colonized much of human life.
Here's an example of a mediocre annotation:
Fjellman, Stephen M. Vinyl Leaves: Walt Disney World and America. New York, Routledge. 1992: This book looks at all kinds of things about Walt Disney's world, from toys to decorations, and talks about how it relates to America. It has chapters on the rides, the pavilions, on corporate strategy, and on other topics.
*(Thanks to trabal.org, from which this paragraph is adopted.)