Off the bat, let me say, I enjoyed the movie. The review is going to be critical in parts, but overall, I think the movie was a fun summer pick, and a decent superhero flick.
The movie is a fine intro into the Hellboy comics, and a good overview of the world. There's these *things* happening, and this secret group deals with them. Yes, there are some changes (in the comics, I believe Hellboy is known, but not generally believed, by the public.) Myers was a new character, brought in to cover the necessary exposition for the audience, and that was kosher. Kroenen was also new, and pretty straightforward. We were missing a couple of the BPRD staff (like Roger the Golem) but that's fine.
The one thing they changed that I really wish they had left in was, in Hellboy's origin. When the soldiers found him in 1944, with them was the Torch of Liberty*, a Captain-America-like superhero. We never find out much about him, but he is the one who gives Hellboy the really cool gun. It's a tiny detail, I know, but I would have liked to seen it added. As it would also establish the setting a little better, IMHO. Oh, and I would have liked to see a reference to Lobster Johnson, but that's just me.
So, back to the actual movie. I think Mignola's art and story carried over pretty well. Some things were *directly* from the comics (carrying the corpse) which went a long way. A couple people have mentioned wanting more Nazi occult stuff, which is fine. I think the Lovecraftian reference is a better one. Hellboy isn't all about nazis, but it's a big pat of his history.
Oh, and the pancakes? It's also right from the comic books. He looooves pancakes.
If nothing else, it means in a month or so I will be able to pick up a few Hellboy shirt and paraphrenalia on the cheap. Heh.
And did you catch the cameo by the Devil's Backbone?
* - I might have his name wrong, I don't have my book in front of me.