Does a large national debt affect the long term value of a small investor's worth, if not invested in commodities?
Not a lot, directly. Your biggest direct risk is that you could buy the debt, and buy it at too high a price (i.e. too low an interest rate) and not make as much money as you ought (and maybe not enough to cover inflation, especially if you buy long-term bonds at low interest rates.)
The indirect risks are mostly that the debt could weigh on economic growth:
- If the government needs more resources to pay off its debt, taxes could rise. This takes money out of the private economy, depressing economic activity and also diluting the incentive to invest (making it harder to sell your holdings to some rich jerk or hedge fund at a high price).
- Government spending on other things could fall. (Lower government spending may have positive economic consequences in the long run if the spending isn't worth the cost of the taxes to pay for it and taxes are cut as a result, but even wasteful spending helps the economy more than spending on debt service.) This is especially a risk for you if your business gets money from the government, e.g. defense contractors.
- There is also an economic phenomenon called "crowding out". When the government holds a lot of debt, it is competing with private businesses and individuals for the attention of lenders, making it more expensive for those businesses and individuals to borrow money. This damages their ability to borrow money for purposes which could grow the economy.
There is also a question of monetary policy, inflation, and interest rates set by the Federal Reserve. Theoretically the government could be tempted to keep interest rates low (to save money) and buy its own bonds ("printing money"), which could cause inflation. Theoretically, they shouldn't, as price stability is one of the Fed's primary mandates. But if they did, inflation makes everything less predictable and is generally obnoxious, which makes everything more risky and drags on the economy. Also, if the nominal value of an asset rises due to inflation, you will likely need to pay taxes on that at some point if you sell it, even though its real value is the same.