[Christ]

Switching, most notably. When/how to switch it on and off.

Other problems would include, but not be limited to:

When the power steering comes on, it's an instant thing, so you go from bull-head steering to "friendly little fairy" steering, and could over steer very easily.

Convincing those who don't know better to leave the damn switch alone.

Remembering to turn it off, so that you didn't just waste all that work for nothing.

Designing the more complex parts of the system, like if you wanted variable pump speed based on load, or switching the pump automatically when the steering reaches a certain point and the MPH is below X, etc.

To relate the setup to something else some of us are familiar with, the most basic way to do it would be like 1-wire Vtec swaps in Civics. While it's great for saying you've got "Vtec, y0!", it's pretty much worthless for real-world circumstances. It just needs a control system more accurate than "Oh crap! I better hit meh VeeTek switch so'z I ken beet diz foo!"