Mining pools and mega farms..
I think the code should have something to disincentivise the centralization of mining. I don't think rules that disallow it would be the answer. More like built in conditions that the free market will turn into decentralization rather than encourage centralization as it is.
This. Maybe it is not a concern right now but it will be soon. Don't think Satoshi imagined that mining will become like this. But I don't think there is a solution to this now.