The primary clinical feature of mild dehydration is thirst. This occurs early as the body's natural response to fluid loss, signaling the need to increase fluid intake to restore balance. In mild dehydration, the body has not yet experienced significant fluid deficit to cause more severe signs.

Dry mucous membranes (Option 1) are typically seen in moderate to severe dehydration, indicating a greater degree of fluid loss affecting the body's tissues.

Hypotension (Option 3) generally develops in more advanced dehydration, when there is a significant reduction in circulating blood volume leading to low blood pressure.

Tachycardia (Option 4) can occur as a compensatory mechanism in response to hypovolemia but is more commonly associated with moderate to severe dehydration, not mild cases.

In summary, thirst is the earliest and primary indicator of mild dehydration, reflecting the body's initial attempt to correct fluid imbalance.

Reference: Ganong's Review of Medical Physiology, 26th Edition, Chapter 2, Page 25