I was a bit underwhelmed that they used the same "the ship flies into the screen and ends up in an animated world" gag as in the previous movie, but the "Yellow Submarine" was pretty and funny, so I warmed to it. Nice to see a lot of one-off characters. Oh, and the title is great, of course.
I mean what I say about the start being show quality. Sal, the demolition derby, Bender getting into D&D, all very nice moments. The asylum scenes were hilarious (and yay, Roberto! He's one of my favourite minor characters). I liked the psychiatrist (I can't remember if he was in "Insane in the Mainframe". If not, he's the first movies-only character I consider classic. EDIT: I'm informed that he was, and thus isn't.
I got my wish: more scenes focusing on the kids! I've never played D&D, but those scenes felt true to life.
There was also a marked lack of gross-out jokes compared to previous films: OK, there were quite a few, but at least this time they just *talked* about it, rather than, say, showing someone eating Fry's over-saccharine blood on a pancake. The only gross-out joke that felt actually gratuitous was Zoidberg cutting a hole into the Professor's abdomen. This isn't Season 11 Simpsons, people.

Even so, that wasn't too bad.
I've never really cared for Mom's sons, but they were *good* in this one. Their Three Stooges antics were laugh-out-loud funny. "I was an owl exterminator!" And the revelation about Igner (even though I saw it coming a bit) was the one thing in this film that actually touched me emotionally. Particularly their clever plot in the end. That's not a problem, really: while the previous two films had their dramatic sides, this one was very clearly a pure comedy/action frolic. And what's wrong with that?
Characterisation was good (barring the Cornwood sequence, which was fairly caricatural, as has been mentioned). Leela understandably spent a bit too much time being angry (irony!), but not overly so. I think Bender was the true star of this one. Liked the Professor, too. He was the other true star. Mom made a good villain (yeah yeah, you know what I mean).
And finally, we know why the Professor hates Wernstrom so much!
Just generally, plenty of funny stunts and one-off gags.
And now, the downsides.
WTF... Nibbler's just back? Without any fuss? OK, so they did have Fry make the "you forgot to wipe our memories" line, which mollified me a bit, but still, that bugged me. And while at the subject, Zoidberg's parents are dead, dammit! I know that was just a random joke, but if the writers don't bother to stick to *their own* canon, what's the point?
The Nibbler farm backstory bothered me, for some reason. It didn't have any continuity errors (that I noticed, at least!), and it was nice to get a bit of filling-in on what happened on Vergon-6, even though that was hardly news, but... I just had no idea that millions of Nibblonians were supposed to have gone missing. I just assumed they all lived on Eternium. It all felt so... random, I guess.
Some jokes just fell flat. No bad writing, just... not up to the sharpness I used to expect from Futurama.
Ah, the fantasy plot. As a LotR parody, it wasn't as awful as I feared, but nor was it particularly inspired. Nice character designs (love the three sons as demons!), some good jokes, and I was pleasantly surprised at the lack of bad fantasy webcomic elements (you know, the transgender centaur ones). The satire of the Noble Pacifists/"I'll never fight again" plot elements was nice, but went on a bit too long. I was half expecting them to do a Terry Goodkind parody and have Leela slaughter all the centaurs like the hippies they are. Heh.
I wasn't so bothered by Amy's sluttiness, because, let's face it, that wasn't "our" Amy. The Cornwood characters were all fairly flat, as has been mentioned.
Officer 1BDI neatly outlines all the problems I have with the Cornwood section, its reality/unreality and the way the characters perceived it, so I won't repeat it here. In general, I think it could have benefitted from some more intellectual writing, making it an overt mirror image of the real world (the way they seemed to be going with Mom making her speech about Igner in both worlds). Surely they could have cut some of the excrement jokes and dwarf-eating to make room for that.
Honestly, it wasn't awful, but when the highlight of a "Lord of the Rings"/D&D/generic fantasy parody is a spoof of the Most Parodied "Star Wars" Scene of All Time, that's a bit weak. (And yes, I mean it, the "Star Wars" parody was the highlight of this film for me. I don't know whether that reflects poorliest on me or on the film.)
The very very end was... words fail me. I'd be a bit interested to find out how they're going to deal with that problem in the next film, but I'm willing to bet real money that they'll be retconning it. Again.