Where is the Line Between Flattering Imitation and Legal Plagiarism?

Some movies are remade after a few decades to bring a story to a new generation. Some of these movies are successful. Others ought never have been made. One of these remakes was the 1997 made-for-TV version of the 1957 classic 12 Angry Men. I have not seen the remake, so I guess I can't slam it too much (not that lack of knowledge has ever stopped me from expressing opinion before), but I am a fan of the original film version. The original movie was nearly perfect, so I don't see any reason for it to have been remade. It seems to me that it is just an attempt at profiting from old success. 12 Angry Men tells a timeless story of integrity and humanity. How can you improve on perfection? Well, I can't think of any other movie remakes that weren't necessary except for Miracle on 34th Street, Annie and Charlie and the Chocolate Factory (you know what I'm talking about, I don't remember the exact names of the Gene Wilder and Johnny Depp versions). Each of these movies were sufficient in their original states. This is a pretty subjective topic though. And a case could be made that the newer Chocolate Factory with Depp as Wonka is more accurate based on the book, which I haven't read, but I'm going to stick with my preference for the Wilder version. Anyhow, flattering imitation or legal plagiarism?
This process also shows up in television when a popular series is turned into a movie. This line of thinking has nothing to do with my original intent behind this posting, I'm just using it as a segue to mention something. I thought McHale's Navy was a remake of a movie, but it is a movie version of an old TV show. So I had to change the subject slightly so I could say that anything with Tom Arnold is unnecessary. That being said, we return to the intended topic.
I am a romantic. Whatever that means (according to the second definition of the word romantic at dictionary.reference.com it is to be fanciful and impractical). I like the movie You've Got Mail. I think it is a nice story. I don't know how relatable it is to me; I'm not a rich bookstore owner like Tom Hanks' character, nor am I an out of work former little bookstore owner woman like Meg Ryan's character. Regardless of being able to directly relate to the characters, I like the story. I only learned recently that it is a remake of the movie The Shop Around the Corner. If you are familiar with Got Mail you will know that Ryan's store is called The Shop Around the Corner. Now that name makes sense. I finally watched the original movie last night. I like it.
Jimmy Stewart played Tom Hanks (use your imagination, you'll follow what I'm saying) and Margaret Sullavan played Meg Ryan in the 1940 film. The story had some subtle differences. Strike that. The two stories had some subtle similarities, but most of the details were entirely different. Could you watch one and then the other and not realize they were the same story? Of course not, but you can watch one and then the other and not feel like you just watched the same movie twice. Both are good, but The Shop Around the Corner is probably a little better.
I would say that in the battle of Hanks/Ryan vs. Stewart/Sullavan, the verdict is flattering imitation. Whereas with 12 Angry Men, the verdict is legal plagiarism. Since this is my blog, I am not going to give evidence for either declaration. I've grown tired of typing at the moment.