It contaminates and clogs ventilation, water supplies and drains. Power often goes out during and after ashfall. Long-term exposure to wet ash can corrode metal. Ash accumulates like heavy snowfall, but doesn't melt. The weight of ash can cause roofs to collapse.
Wet ash is slippery.
Ash resuspended by wind, and human activity can disrupt lives for months after an eruption. In ashy areas, use dust masks and eye protection. If you don't have a dust mask, use a wet handkerchief. As much as possible, keep ash out of buildings, machinery, air and water supplies, downspouts, stormdrains, etc. Don't tie up phone line with non-emergency calls.