
I wish I could remember this to credit it properly, but when Attack of the Clones came out, I recall an observation to the effect that this was the first film in history to get good reviews simply for not being awful. And if that's the case, then I think The Muppets is the second one.
The fact that it even occasionally, for brief moments, manages to recall the brilliance of the Muppets in their heyday is probably a minor miracle (and is to be celebrated). But there's a constant drag on the proceedings, what feels like a pervasive, all encompassing lack of faith: the Muppets aren't really special, they can't carry the film, we need to have all this other stuff in it, and they end up playing second-fiddle in their own movie to people/things who are not anywhere near as interesting. It's just like "Puppet Show and Spinal Tap" except, ironically, Jason Segal is the puppet show.
Like the whole running...I hesitate to call it a 'gag'...about the 80's stuff, which seemed wildly misjudged/out of place. The Muppets were never of their time. And if anything their 'time' was the mid/late 70's, back when variety shows like The Donny and Marie Show were still on TV. Geez, it's not like it was Knight Rider.
The most important thing for me, though, is that something feels off about Kermit's characterization. Part of what's important about Kermit is that he always represents the best of us, the best of humanity. Yes, he occasionally loses his temper, has moments of doubt and crisis, etc., but at the end of the day he never gives up and he always does the right thing. So what's supposedly happened in the intervening years doesn't feel right, nor do his initial scenes with Walter, Mary and Gary. He seems defeated and quasi-broken, and that's not Kermit. And then at the end of the telethon, when they fall short, Kermit's like "Oh well, fuck it. Let's throw in the towel." And that's not him either.
I'm sure that part of it is the fact that he's not the protagonist, so it can't be him that takes the "come on, you can do it" line. That now has to come from outside, from Gary/Walter. And another part of it may be the perceived need to "update" things, where "update" means make "darker" and "edgier" (though we're not talking about The Dark Knight or anything obviously). But I think that just underlines the fact that those decisions were a mistake.
Again, it's not that it's awful, and there are some really good moments. But I think Frank Oz was right in a lot of what he (and others) said, even if I don't feel it quite as strongly. However much of a fan Jason Segal apparently is, the script doesn't entirely respect the characters. And so much of it just feels so generic, and bland, and uninspired and safe (including the songs, which I find desperately banal), and that's 100% not what the Muppets are about.
Now lest anyone think that I'm just a bitter old man who remembers a time when stuff didn't suck, "Muppets Tonight" is absolutely tremendous -- fully the equal of the original series. It's a crime against humanity that that's not out on DVD (along with Seasons 4 and 5 of The Muppet Show). (TOS is better than TNG though.)
For the next hour, everything in this post is strictly based on the available facts.