It really depends on the what's being adapted. With comic book adaptations I'm very lenient because they're trying to pick and chose from 40-70 years of stories and make a 2 to 2:30 movie from that. As long as the characters spirit remains the same and the situations make sense I'll like it. I thought Wolverine sucked because it failed at both of those, my feeling about the movie can be boiled down to two things, the Gambit/Wolverine fight, they're both at one end of the alley, the person they're both pissed at, Sabertooth is at the other end, so what do they do? Fight each other. The other thing is Deadpool, Ryan Reynolds was perfectly cast for that role, but you take the best part of the character, his mouth and sew it shut?!?! No, I'm sorry that's unforgivable, say you'll fix it in a spin off all you want, but it shouldn't need fixing in the first place.
In terms of Harry Potter movies, I think too many of the big plot points have been cut, ever since the 4th movie, the directors have been obsessed with the sex lives of these characters to the point that I think Chris Hansen should visit the set. The movies are about the kids growing up, yes, but the major narrative is the return and eventual defeat of Voldemort.
I think movies based on books should stick fairly close to the source material, sure for some longer books, things need to be cut, but for the most part you should be able to stick to the book. Some movies can get away with it though, the Bourne movies are a perfect example, good movie, but bare very, very, very little resemeblence to the source material.