It's a common occurrence in comics, rebooting or restructuring a team. Members die, leave, marry off, or etc. to be replaced by news members. It helps to bring in new readers and add new dynamics to books that may be getting a little stale.
Having said that, I really like the line-up of the newest incarnation of the Justice League of America. A few years ago, DC Comics realized that for the JLA to work, it needed the "big guns" of Superman, Batman and Wonder Woman. There were several failed or lackluster attempts to have a league without those characters and it just felt like another team book and not the JLA.
Now for a little commentary on the characters selected (working clockwise, from the right):
Minus the big three, the rest of the line-up is nice. I like the addition of Red Arrow (formerly Arsenal or Speedy). One of the original League's members was Green Arrow and having his side-kick take his role in this new versions plays upon DC's history and shows that time does pass–to an extent–in the DCU.
The new Hawkgirl (Kendra Suanders) is also a welcome addition. The Hawkman solo title switched to Hawkgirl and has been horrible for about a year now, so it's good to see the character being written well in a good book.
Red Tornado and Black Lightning are two off-and-on League members that deserver a good shot at the big leagues. I'm interested to see how they resolve the current storyline with the Red Tornado becoming human. Maybe Black Lightning will get a new name to go with his new status quo.
Vixen is another great choice. She was given the spotlight on a few episodes of the JLU animated series last year and with an increased role in the League and a re-definition of her powers, she could be the character with the most to gain.
Hal Jordan, the Silver-Age Green Lantern, hasn't been a part of the League since the 90's when he was in the Justice League Europe title. The character has been through many changes in the past decade (from villain/mass murderer to dead, spirit, avenging ghost). A 2005-2006 revamp and return has brought the character hack to his roots as a fearless Air Force test pilot, but left him at odds with Batman on a few occasions.
Skip over the big three and end with Black Canary. It's the only character that I'm a little disappointed to see. Not because I don't like her, but I really like where they left the character at the end of her 100-issue arc in the Birds of Prey series, and I just don't see that lining up in the JLA. I've heard rumors of where she may go in the future, so we'll see how it all plays out.
Overall, I'm really liking the new incarnation of the Justice League. It moves a little slow compared to my other favorite title, Justice Society of America, but it's grand scale super-heroics done correctly. I could also do without the Ed Benes art. I'd like someone more "comic-book" style or classic style. It's not stopping me from enjoying the book. Having said that, the art at the top would be a great poster. Nice way of showing all the previous versions of the League, while spot-lighting the new version.