I went into this expecting to see a fun, mindless action movie... maybe a little bit campy or predictable, but still entertaining.
Unfortunately, it was the opposite of entertaining. It was a mess of things that didn't make any sense, characters as plot devices, and terribly unfunny one-liners. Oh, the one-liners. They were so cringe-worthy. At one point, Hellboy says the phrase "your royal ass-ness!" to Prince Nuada as if it's some sort of witty insult, rather than something that nobody has said since fifth grade.
Also, Seth McFarlane as mechanical-suit-man was incredibly distracting because his voice, with the German accent, sounded exactly like Klauss... the fish from American Dad.
None of the plot developments made any sense in the context of the story; they were only there to serve the writers' ends and seemed terribly contrived and unrealistic. And OK, movies always have plot holes -- but in most cases, the movie manages to suspend your disbelief and you forgive its minor flaws. Not so in this case, because there were just so many problems.
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The Abe/Prince Nuala romance was stupid and sudden and utterly pointless; Abe was useless as a member of the team (why was he even there??); somehow the elves were content to live in the SEWERS after they had won the war and signed a treaty that humans should have the cities and elves the forests... I mean, really. Prince Nuada was the only one who was like, "uhh.. something's not right here. oh right, we're living in a f*cking SEWER!" And it goes on: Abe gives up the crown piece to Prince Nuada even though his hostage (the princess) isn't in ANY danger whatsoever; for some reason, Princess Nuala sacrifices herself AND her brother to save the life of Hellboy, who she wouldn't even care about if it weren't for the fact that he's the main protagonist and the audience is supposed to be rooting for him; Selma Blair's character tells Hellboy that she's pregnant to give him a reason to live (this whole scene was about as subtle as a brick. and honestly -- loving your girlfriend isn't reason enough to live?); and then everyone inexplicably QUITS the team for no reason (except, I guess to seem like Badass Rebels That Don't Bow to Authority).
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None of this came off as believable to me; I was constantly pulled out of the movie because it only made sense in the context of del toro trying to move the plot along when the events constituting it just wouldn't fit.
The only redeeming part of the whole thing was Prince Nuada, who was sinister and gravity-defying and interesting to watch. I found myself rooting for him to stab the sh*t out of Hellboy.