How much should I put into my 401k?

I’m under 30, make a pretty good salary, have no significant debt (other than my house), how much should I be putting into my 401k? My employer doesn’t match my contribution at all, but they do have a profit sharing plan so a percentage of my salary goes into my 401k regardless of my contribution. In other words, I’m getting no “free money” from my employer by putting my money into a 401k.

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The tax implications are obvious, but what about the opportunity costs? By tying all my money up in my 401k, I won’t have access to it (without penalty) until I’m 60. There has got to be a significant opportunity cost of tying up a significant chunk of my savings for the next 30+ years. Aside from the allowable “hardship withdrawl” reasons that won’t be penalized, what if I come across a great business or investment opportunity where having an extra 20, 50, or 100 thousand dollars may really make a difference and signficantly alter my financial life by the time I retire? If most of my savings are in my 401k, I won’t have access to that money without being forced to pay income taxes and a hefty 10% penalty.

Socking away money for retirement is definetely a good idea, but doing it all in a 401k account seems like its severely limiting my liquidity, preventing me from being able to quickly make investments when opportunities arise.

So, what do you think I should do?