Structural problem for rotating colonies
Think magnetic levitating railways. The inside of the colony rotates and the loads are transmitted along a high-speed repulsion interface to an outer shell that is static, and can be as thick as you need it.
I don't particularly like it because it's not fail-safe enough for my thinking, although a LOT of redundancy is possible. Power is required, too.
Personally, I wonder if it might not be less difficult to adapt humans to zero-gee.