Although a great Sci-Fi show, they've deviated too much from what Star Trek is supposed to be about. They've gone the route of the lastest JJ Abrams movies, and made the story mostly about the action. Star Trek shows are traditionally driven by intense realism, philosophy, and dialogue. Here they've thrown the realism out the window in many places, which screws up how this show is supposed to fit within the Star Trek canon.
It might be more popular with people that don't normally like Star Trek, but regular long-time Star Trek fans are left disappointed. Not with the story, per say. It was very entertaining. But the fact that there seems to be little dedication to what made all the other Star Trek shows so great and enduring: The fact that they are so real. The point of Star Trek has always been to be as realistic as possible. The crew react in realistic ways to whats going on around them. It seems this show has basically disregarded that dedication to realism. It's in the Star Trek universe, and Star Trekky things are happening, But instead of realism its all about the action and the suspense.
The technology introduced in this show doesn't even make sense. We can suspend disbelief for Warp Drives and sub-space communications because those things might even be possible in the far future. But what they've added in this show, I don't want to give it away, but it makes no sense whatsoever.
And that's really what Star Trek fans don't like about the show. Instead of hard realism of science, crew reactions, and situations, we have a much less realistic space opera. Don't get me wrong, I liked the show a lot. It was very entertaining. Buts not really Star Trek on the inside, just on the outside.