I'm not jumping for joy over this (not being a Whedon hater), but I'm not broken up about it either. I'm not saying that Whedon sucks, but from the very beginning I've had the feeling that he was a "name" pick, someone who became famous for other work that is only somewhat similar to WW (Buffy has a strong female lead), but may not necessarily be the right man for the job. He seemed like someone who was picked to please certain fanboys, and who wanted to stray too far from the source material and go off in his own directions (no "star spangled panties," no flying, original villain).
I would rather wait a couple more years and see the movie done right than to have it rushed out. Batman Begins was worth the wait.
I DON'T want to see some name director get picked only to screw the movie up produce a movie that is not as great as it should be. Ang Lee is a talented director, but he completely dropped the ball on Hulk. The movie was overly talky, and turned off fans with a number of stupid decisions (convulated origin involving mutation and nanites, 15 foot Hulk, Absorbing Dad). Marvel/Universal's big tentpole movie barely made its money back domestically, and the Hulk brand name was severely damaged. They're making a new Hulk movie (basically a restart to the franchise), but it's not going to have the hype and expectations that it would have if the franchise was still fresh.
Superman Returns was a good movie. The reviews were positive (but not great, only around 75% at Rotten Tomatoes), but the box office was very disappointing. Singer did a good job on the X-Men movies. However, I thought that he was WAY too much in love with the Donner movies. Fans wanted a fresh, modern take on Superman. Besides, making a pseudo-sequel to a decades-old movie (while ignoring the last two sequels), set 5 years after Superman went into a self-imposed exile that you made up yourself is NO way to introduce a new generation to Superman. Singer also ignored the positive developments in Lex Luthor's character post-crisis. When you throw in Super-kid (wtf?!), who will be baggage that can't be ignored in later movies, you have a Superman franchise which despite being worthy of the name, falls short of the greatness it should have achieved.
Going back to the original topic, I don't see this as a huge loss. While it's not guaranteed that the new director will be more respectful to WW, at least now there's a chance. A lot of Whedon's decisions worried me, and as I said before, I would rather wait and see the movie done right.