It comes down to how/why it's done. The new Planet of the Apes movies are another example of doing it right. They've done something new with it, been respectful to the original, and had thought put into them. Compare it to the Tim Burton remake, which was completely soulless, and was an empty movie that missed the mark.
The difference is remaking/rebooting something for an idea, rather than for a name. I wouldn't say the original version of The Fly was really a hugely popular thing by the time they remade it, unlike Ghostbusters which is still beloved today, but a man turning into a fly from an experiment gone wrong is an interesting idea, and didn't have a lot of other elements to capture or lose in the process (I've seen the original, but not the '80s one, strangely enough).
I'm now feeling an itch to rank the best reboots/remakes, DTF style.