They become distractions. If you watch for it, you can sometimes catch people doing a vague imitation of penguins, with their hands flipping about at their waists.
Finally, tap your own natural style, but then expand on it. Try doing what feels "natural" and then make it bigger. If you don't naturally gesture at all, start by just adding one or two gestures to a sentence you practice in private. Try using words that have obvious gestures associated with them (large, tall, small, all, etc.). It probably will not feel or look natural at first, but, with a bit of practice, you can build on your innate gestural vocabulary to become a more powerful and eloquent speaker.