1. Strong female characters. This may not be the top priority for some people, but the Justice League has more than its fair share of badass women. Although the original roster only had one female, Wonder Woman, the team has since expanded to include almost every superheroine: Huntress, Hawkgirl, Black Canary, Zatanna, and so on.
2. There's something for everyone. Because the Justice League encompasses so many characters, lots of personalities are represented. Obviously, the film wouldn't include heavy screen time for many characters, but the diversity of the team really makes a difference. Not every member is sarcastic, handsome, or even human.
3. It ties everything together. For years, DC has been producing movies for Superman, Batman, and even the Green Lantern (with some false starts on a Wonder Woman show/film), but everything has existed separately. By combining the efforts into one movie, DC can carry the momentum from the Justice League into each of the separate ventures. After all, there won't be a new Avengers movie every year, but new films for the individual characters can be peppered throughout a few years to keep the steam going.
4. It appeals to a wider audience. For example, my hatred of Superman runs pretty deep, but I'm always willing to watch Superman television episodes if there's some Batman material involved. A Justice League movie probably won't change my decades-long disdain for Kal-El, but it may convince me to sit through a separate Superman movie I wouldn't have watched otherwise.
5. The stage has already been set. There's certainly no shortage of source materials for a Justice League film, and the Justice League television series has won numerous awards. Likewise, there's an active Green Lantern/Hal Jordan (albeit not well received) and a new Superman on the horizon. Forward momentum is building, and DC needs their next big thing now that the Dark Knight trilogy has wrapped up.
Notice that I did not include "to beat Marvel" or "to compete with Marvel". The better superhero/comic-based films do, the better for the entire industry. We don't want more Elektra or Daredevil-types of movies; we want witty, engaging films that reward our knowledge of comics while still appealing to non-comic book audiences. DC has produced some excellent material, and the time is right to push forward with the Justice League.