I got that feeling too after finishing the first season. But the second season surprised me. I was not expecting it to be that good.
I'm more than halfway through season 3. It's definitely moved a lot slower, I admit. But I'm still enjoying it.
By far, the show's biggest weakness is the amount of time not spent following Dexter. Necessary, but boring. His sister was cute for the first two seasons, but her personality's really starting to agitate me.