There is just something about this movie that makes it so great. There's nothing truly outstanding about any one aspect of the movie; nothing that will blow you away. However, everything is put together perfectly.
Personally, I have a theory that any movie that absolutely blows you away the first time you see it, will lose its entertainment value very quickly. Not necessarily because the movie was not good, but because it just seems to get old too fast. This is NOT one of those movies.
This is the kind of movie that will be enjoyed countless times. The dialogue is slick, and always to the point. There are few (if any) overdone scenes and/or lines that tend to become annoying over time.
This movie flows exceptionally well. Personally, I couldn't quite figure out why until I watched the "behind the scenes" footage on the DVD. I didn't really notice all of the work they put into using as few shots as possible. The extra features on the DVD really explain it all. Almost every scene uses considerably less shots than you would think it takes to convey all of the activity going on. The music punctuates the flow of the movie and the shots extremely well.
The cast is almost all big names. It's great to see actors like Pitt and Clooney and Damon come together in a movie and interact so well, without having to be the pretty boys or bad-ass heroes of the movie.