My biggest fear is that in 30-40 years the US government and/or 401k funds are so poorly run that they tax 50/60/70% of the income (withdrawals) out of the 401k to sustain theirselves. Lots of people tell me "that won't happen" but when I study history, I realize that the only people who think that are ones who have never lived through a major war. Wars = higher taxes. To me, putting money in a 401k is trusting that everything will be ok for a long time to come, and it's just hard to justify that the way history books show the cycles of war.
It scares me so much that I'd rather pay the taxes now, and invest as I please.
Edit: as omouse mentioned, that is my other fear. That through technology and healthcare innovations, as well as the government recognizing the need to "adjusting" the retirement age, how do I know I will get my 401k at 65? What if it is 85? That's too risky for me :/