If i try to load a older minecraft world in the newest snapshot, I only see the Loading world text without anything happening, but I can create a new world and the world does load.
Environment
Windows 10 64 bit
GTX 1060
I7 6700k
16GB RAM
Java 8 Development Kit
Does the game crash, and is there a crash report in (minecraft/crash-reports/crash-<DATE>-client.txt)?