[Mulch]

EDIT: Complete do-over since my reasons at the time were completely stupid, plus I have time to kill. I realize that no one will ever read this, but I just need to wipe the angry, unexplained list out of existence in favor of one that makes sense.

I Only Have Surprise for You
The show that's always been praised for being character driven does an episode that is entirely script-driven. Chaos ensues.
The problem with this one is simply that it's so forced. The characters are completely sacrificed for jokes. First off, everyone in the house is in on Bloo's evil scheme to embarrass Mac on his birthday. I just find it a tad hard to believe, with the established characterisations, that not one person, not even Frankie, Wilt or Herriman (or even Mac's own mother in the flashbacks), sees anything morally wrong with what they're doing. Still, that factor is a forgivable detail, especially compared to the gigantic, in-your-face problem that completely ruins the episode: Mac.
I mean, they weren't even trying with his character here. Intelligence and common sense, his most obvious character traits, are just pushed aside and ignored. Firstly, there's the point at which he pretends to go home. This is where the real Mac would to the smart thing and actually go home. Instead, Bizarro Mac comes back, hides in the attic and swears to go berserk and ruin the party, which he already would've done if he had just left! Next, real Mac would easily figure out that everyone would have been much more clear about a surprise party for that annoying thing that turns out to be Madame Foster.
The episode even points this out itself, like one of the staff writers realized that it made no sense and tried to explain it but instead wound up just calling attention to the flaw. (I did like the joke that Bloo hadn't thought about that and didn't predict every single thing Mac said, though.) Also, he's the only sympathisable character in the episode, and, despite all those "mean things" he does, is the protagonist and the one you're rooting for, and gets the incredibly short stick in the end.
Then there's Bloo. No problem with his characterisation; he's always a complete jackass. It's just that his unbelievably thought-out plans and gloating attitude make him annoying as fudge. Seriously, there are moments in here where I just want to punch him in the face, particularly the "One. Step. Ahead." scene. (I actually like to think that one second after the episode ends, Mac actually does reach out of the cake and punch him.)
The last huge problem is the fact that the plot, which is the thing that pushes all the characters out of the way in the first place, makes no sense itself! How on Earth does Bloo predict every one of Bizarro Mac's actions? It just... ugh. I mean, there are some funny jokes in there, and I really don't mind character torture anymore, but that's just not enough.

(Wow, I wrote a novel. I'll fill in the four missing episodes later.)