Hello, friends.
I've recently run into a rather frustrating problem that took me a very long time to discover the solution to, and thought I would share since it is a problem I see cropping up in the help forums frequently.

While working with resource packs, I found that sometimes the pack simply would not show up on the list when I opened Minecraft. My folder structure was proper, all the necessary files were there... It simply refused to appear. By copying files from a functioning resource pack, I narrowed it down to a problem with one file: pack.mcmeta.

That still didn't tell me what the problem was. If I made an exact copy of the working pack's pack.mcmeta, the pack would show up just fine- but not with the description I wanted. The second I changed it to a custom description, the pack would stop working. I was about to give up the pack-making scene for good, when suddenly it hit me- every time I changed the description text, my text editor was automatically changing the straight quotes ("sample") to curved, or book quotes (“sample”). It was an easy mistake to miss.

In Textedit for Mac, I solved this by clicking on Edit>Substitutions>Show Substitutions, then unchecking the box beside "smart quotes." (You will need to retype them, if it has already changed them, to change them back.) In Notepad for Windows, according to Claire, you can change between types of quotes by hitting CTRL+SHIFT+".

TL;DR: All quotes in your pack.mcmeta file must look like this: "" not like this: “”

Best of luck to you all, and happy pixel-packing!