The Meaning of Life, the Universe, and Everything.
Quite a bit has changed in Minecraft's code after 1.7.10. For instance, starting with 1.8, all items and blocks must now have .json model files for them. Another notable change, is that at some point after 1.8.9, the names of resource files can not contain any capital letters, or else they will not load.
As you might expect, there are countless smaller changes as well.