1) Soda Sippers/Swishers - low volume soda drinks that consume it slow. Typically people who do this may take 1-4 hours just to consume one can/bottle. The amount of time teeth are exposed to acid in soda sippers can break down enamel rapidly. If you are going to consume a soda, energy drink, or sports drink just consume and be done with it. But constantly bathing your teeth in acid over the long term will destroy them and cause cavities.
2) Brushing Too Hard / Toothpaste Abuse - if you constantly are changing toothbrushes or if you buy medium or hard bristle brushes, or if the bristles on your brush splay out to the sides, you are brushing too hard. The combination of this bad habit + the use of today's abrasive whitening toothpastes can lead to forced gum recession, sensitive teeth, and even enamel breakdown. Seems that in general people that do this have an obsession with a clean feeling in their mouth and that feeling is derived from abusing the gums. There is also a feeling that if they brush harder their teeth will be whiter, when actually the opposite occurs because as you wear away enamel, teeth get darker.
3) Fingernail Biting - Another bad habit that destroys the enamel on the front teeth and wears the teeth down. Teeth weren't designed to chew nails, and over time the nails keep growing back, but the teeth just get worn down never to grow back again.
4) Ice Chewers - Crunching on something hard that is at an extremely different temperature than the body is really hard on teeth. It's even harder on dental work, especially white fillings and sealants.
5) Pen/Pencil Chewers - Just like with fingernail biting, your front/side teeth take a beating when you do this. Doing anything teeth weren't designed to ultimately harms them.
6) Popcorn kernel crunchers - several people out there like to crunch on the slightly popped or un-popped popcorn kernels. This isn't much better than chewing on rocks. These things are EXTREMELY hard and I've seen several cracked molars on people with that habit.