Effective cross-country skiing requires the boot to flex forward to allow a striding action. The boots should be designed around a sole piece that allows forward flexing while still keeping the ankle from moving sideways. The upper portion, the cuff, is soft and designed for comfort and warmth.
A fairly stiff sole for efficient pedalling, but one with enough flex and a rubberlug outsole to allow good traction for walking on slick or rugged rails.
It should contain drain holes and a porous fabric that allows quick release of water. The fabric should also be quick drying. This eases the transition from water to land by giving traction on slippery surfaces. It should have 360 degree ventilation for maximum breathability, and a thick sole to make sure you're protected when you step on sharp rocks that lurk along the bottom.
Sneakers or similar soft-soled shoes work best. They must fit tightly to stay on while youre freefalling at 120 mph. Shoes or boots with hooks risk snagging during your exit or fall.