Making a good poster

Im currently doing a project for school and what my classmates and I are trying to figure out is "what makes a good poster?". On my opinion I would say that a good poster would depend on the theme and the type of audience its trying to attract, not everyone is alike. Also i would say color is very important, you dont want something that will just scream in bright colors and irritate your eyes and you would'nt want something too dark that makes it difficult to catch your attention. Thats as far as I can think would make a good poster. Comments and opinions will be greatly appreciated.