There is a simple test for what is "equipment".
Simply enough, is the item completely under the control of the character? In this example, a set of handcuffs would be completely under the control of the character (that he can't take them off is an irrelevant distinction). But if the cuffs are chained to the wall, then they are not entirely under the character's control and they are not equipment.
This same test is useful for determining how other spells, such as Invisibility, act. There are some edge cases that are hard to cover: for example, a fisherman's rod is certainly under his control and qualifies as equipment, but the bobber floating slack on the water are not entirely under his control, and the fish caught on the hook underwater is definitely not under his control. There is no perfect test that will handle every situation, which is why God invented DMs.
For a Druid in handcuffs - the handcuffs change with him, which would allow him to run/swim/fly away, but when he returns to human form he would still be wearing them.
A Druid chained to a wall - the chains would not change with him, as they are not entirely under his control and do not qualify as equipment. If the druid picks an appropriate form, he could slip out of the chains.