Ever since the ability to select multiple resource packs was added in snapshot 13w36a, the ESC key no longer functions like it used to. Before this change, pressing ESC after you were happy with your change of resource pack would close the menu, save changes, load the resource pack and rerender the game accordingly. Now pressing ESC simply closes the menu without applying your changes, which can be annoying when you're used to using the ESC key as a shortcut to leave the in-game menus.

I was surprised that nobody else beat me to reporting this, as every person I have met playing Minecraft has been aware of the ESC key's usefulness. I didn't search for a bug report on this problem until recently, so that explains why I'm so late in reporting it.

Edit: If I change any other game settings and press ESC (such as the graphical settings or language settings), the game resumes and the changes are applied.

TL;DR: there is inconsistency involving the saving of game settings that didn't exist before the changes from texture packs to resource packs.