Gothic trench coats certainly make an impact: they are much longer than regular trench coats, and some almost touch the ground. In that respect they are more like dusters than trench coats, but the aesthetic is completely different. Dusters are usually made of leather, are often brown or tan, and are associated with cowboys. Weather you choose a model that goes down to your ankles or an even longer one, you may want to make sure that the bottom portion is wide enough so that it lets you take steps comfortably. Not that comfort has anything to do with this fashion, but if you end up having to take small steps in order to move around, then the effect is ruined.
Two examples – pretty cool looking, but not my thing:
Mens gothic trench coats pictured:
Left: Necessary Evil Mens Highwayman Full Length Coat
Right: Necessary Evil Odin Goth Full Length Black Twill Mens Hooded Coat
The sleeves of a Goth coat should also be longer than on regular coats. The coats are also usually equipped with a hood. As for zippers, buttons, hooks and belts, they are a big part of the overall look.