I have only seen it once and that was a while ago. Bringing up this topic kinda makes me want to watch it again just to remember it. The thing is, we are introduced to a completely new group of characters. A young Kermit and younger versions of Statler and Waldorf are the only familar characters we see. That could be one reason why it can't hold some fan's attention. I think it is cute, and if there is anything great about the film, I would say it is the concept. Since I first heard about KSY, I thought it was a clever idea. Kermit is such an icon and symbol of our American culture and I think we all wonder what he would have been like as a child in the swamp.
Another thing is that I think this particular film may have been aimed at a much younger audience. Say like 4-10 year olds. While the writing is cute and clever, much of the jokes are very juvinal.
However, I think you should go and see it if you haven't already. I believe that you should see it before you are able to freely critique it. Too many people, especially around here, bash and pick apart things they haven't even seen yet. They just go by what others say instead of allowing their own opinions to form. You might say it is good and you might say it is bad.
So watch it, and judge it for yourself. You may just like it!