Hmm, two apparent ones come to mind:

- Concentration of ‘voting power’: if more investors flock to index funds, a smaller group of fund managers will have more voting power on behalf of their clients.

Theoretically, a ‘concentration of power’ could give [few] fund managers a lot more responsibility than they should otherwise have; that’s a lot of livelihoods to hold be responsible for.

- Increased crorrelation: if a [large enough] portion of the population invests in index funds, the returns of those funds will become more correlated with each other; a ripple could become a tsunami.