Existing cell technology does not support proper authentication. Your mobile device is authenticated, but cell towers are assumed to be trusted and you have no control over what you connect to. This allows all kinds of Man in the Middle attacks, where one could pretend to be a cell tower and capture traffic.
Hypothetical example - you are standing next to a cartoon robber with a big black box called "FAKE CELL TOWER". The only way you could prevent your smartphone from getting connected to this device is to turn data off. Once you connect, attackers can push fake app updates, inject exploits into your browsing, inspect your unencrypted traffic, try to get you accept compromised certificates, redirect your traffic and so on.
And this is why I am not excited about LTE direct.