The reason given in the New York Times article about the Sci Fi name change is that "Sci Fi" couldn't be trademarked. Syfy is a wacky made up name, so it can be:
“We couldn’t own Sci Fi; it’s a genre,” said Bonnie Hammer, the former president of Sci Fi who became the president of NBC Universal Cable Entertainment and Universal Cable Productions. “But we can own Syfy.”
I believe a good portion of the rebranding, however, comes from the fact that the Sci Fi Channel wants to move into broadcasting mystery television shows, action and adventure, fantasy, and a variety of other genres that don't quite fit "Science Fiction." I don't quite agree with this. I think most people are comfortable watching a mystery show on a channel called "Sci Fi," and wouldn't be too put out by it. Same goes for fantasy. "Syfy" seems too generic to me, like "TNT" or "Spike." And I don't want another generic TV station. I liked the Sci Fi channel because it was the only one of its kind in its niche, and I really hope it stays that way. (Although I didn't like Sci Fi because I thought they didn't always take the best care of their flagship shows.)
I will say that I'm a fan of the new tagline. "Imagine Greater" captures, to me, what the station is all about. "Imagine" has science fiction and fantasy connotations, which is great, and the whole phrase promises the viewer something more, something better, and something that they haven't before encountered. Works for me.
Overall, it's not too bad a change. It beats the Pepsi and the Tropicana rebranding, at least.