I think this poster is succesful because of its use of repitition of color. The red, yellow and green work nicely together, and they also are associated with Jamaica which is where the marathon is. The author used alignment of the text which works well to move your eye through the poster, and the seperation at the bottom of the page is very succesful because it allows a change for what is being talked about; what the event is at the top, and information and sponsors at the bottom.

http://imagefactorylimited.com/images/ReggaeMarathon_Poster_2003.jpg