In my experience, many critics fall into the unfortunate habit of being way too hrash on a lot of pictures. On the other hand, I've also read reviews which seem way too lenient in some respects. That's why I generally don't trust a lot of critics: their reviews are often shaped more by personal opinion than professional expertise, and that can be more of a detriment at times, depending on the reviewer.
While it's certainly true that most departures from the source material have yielded disastrous results, there's also been cases where it worked for the better. "The Mask" bore little resemblance to the comics in its final form, yet it's widely considered one of the best films from Jim Carrey's early career. It's also notable for being the very first on-screen role for Cameron Diaz, who was a professional model before playing Tina Carlyle.
My biggest beef with this situation is when a filmmaker completely ignores the elements which made the original book such a success. It's one thing to make a film based o the "spirit" or "essence" of a character, but to ignore those attributes completely insults the fans. I haven't seen the "Hitman" movie, but from what I've heard it's nothing like the games, instead being filled with tons of blood and sex that the games avoided.
In recent years, I think both "X3" and "Spider-Man 3" fell victim to this, the former likely more so. Films which explore many characters at once need the appropriate amount of time to do it, "Lord of the Rings" being a prime example.
If the source material is more serious than funny, then the film can greatly suffer if it's taken the opposite way...and vice versa. As for casting, there's very few instances where I've been against a superhero casting choice. Jessica Alba was watchable as Sue in the original "Fantastic Four", but her performance in the sequel was not for the better. She came off as manipulative and rather condescending, with way too fierce a feminist streak. Her passive-agressive crap made me very uncomfortable. Similarly, I don't think Ray Stevenson is a particularly good choice for the Punisher; he seems too dark from the trailer, even for a character of that nature.