I've got my 401/IRA entirely in the Vanguard Total Stock Market fund. Retirement is still a couple decades away, and for me having bonds in the portfolio is just going to hurt my long-term gains in favor of lower volatility in the shorter-term. I'll shift the allocation in like 15 years or so when retirement is closer.
I don't think your advice is to @Mr Bill in particular, but I did want to address it in case @Mr Bill is confused by it.Word of advice, prolong decisions is the worst mistake you can make. Do it now, diversify into different categories - small cap, mid cap, large cap, emerging markets . You wait 15 more years than you have the potential to miss out on gains. Retirement doesn't need to be decades away. Think and practice to get to FIRE ( Financially Independent Retire Early )
The Vanguard Total Stock Market Index Fund is an all-cap domestic (U.S.) equity fund, so it contains small, mid, and large cap at market weight. The diversifications that @Mr Bill might consider are:
1. International - allocate between zero and forty percent of your equity allocation to international equities. My recommendation would be the Vanguard Total International Stock Index fund, as it is a low cost, broad-based index fund that contains both developed market equities and emerging market equities, and also contains large, mid, and small caps. There are several acceptable strategies for determining what you want your allocation to international to be - all the way from "I don't want any" to "market weight." I allocate based on minimizing risk, so I end up at about 20% international.
2. Bonds - it's usually the case that even if your investment horizon is 40 years, your recommended allocation to bonds is at least 10%. The small drag on your portfolio return is somewhat mitigated by a small return from rebalancing when stock prices take a dive. The main reason for a bond allocation, though, is to keep you from selling stocks during a downturn. That said, I don't personally have issues with someone like @Mr Bill with a long investment horizon being 100% equities.
One last piece of advice: If you're looking for a good place for your fixed-income allocation, you should look at U.S. Savings Bonds. If anyone is interested I can go into why they're such a good deal nowadays.