sometimes a movie trailer will call out the big names starring in the film. these are referred to as "IDs." studios like to use IDs when they feel the cast is a significant selling point for a particular film. (for some really fun IDs, watch the In Bruges trailer in the previous post.)
an ensemble film will usually include IDs because its vast collection of stars is part of its appeal. for example, check out the Valentine's Day trailer. since the film's claim to fame is its star-studded cast, its marketing capitalizes on that.
but sometimes you don't need IDs, even when there's a big cast. the Harry Potter trailers don't use IDs because we already know the actors--and also because the cast has little bearing on whether or not we'll watch the film. other reasons not to include IDs: if the film is low budget and has no recognizable actors; if IDs would interrupt the flow of the trailer (like in Nine, below); or if the cast is SO BIG that there's no need to call them out because we'd recognize them anywhere (also like Nine).
but by far my favorite use of IDs is in the trailer for The Ten. check it out, towards the end. it's pretty hilarious.