(a) perfect emulation of human behaviour makes spam and fraud detection almost impossible. an LLM can now have an intelligent, reasoned conversation over weeks with a target to perfectly emulate an entity known to the target (their bank, loved one, etc etc).
(b) direct attack on authenticity : we aren't far away from even video being faked in real time such that it's not even sufficient to get a person on a zoom call to understand that they are not real
(c) entrenching of ultra subtle and complex biases into automated processes. I expect companies to rapidly deploy LLMs to automate aspects of information processing and the paradox is that the better it gets at not showing explicit biases the more insidious the residual will be. For example it's not going to automatically reject all black applicants for loans but it may well implement some much more subtle bias that is very hard to prove.
(d) flooding of the internet with garbage. This might be the worst one in the end. I feel like fairly quickly we're going to see this evolve to requiring real identity for actual humans and ability to digitally sign content in ways bots can't replicate. That will then be the real problem outcome because the downstream effects of that will enable all kinds of censorship and control that we have thus far resisted / avoided on the internet.