I'd say the main thing to watch out for is the druid's "ground control" spells. While they don't have the offensive punch of wizards or the defenses of clerics, nobody can screw up a battlefield worse than a creative druid. Spike Stones, Wall of Thorns, and even the lowly Entangle can all make problems for bad guys.
Hampering your enemy's ability to move where he wants is a big advantage in combat, especially with D&D's abstracted/miniatures system. It's just tough to counter a druid's ability to control the field without cheesing out and making all opponents fly.