In no particular order:
1. Sabotage the effectiveness of the seatbelt.
This really applies only to taxi drivers. I've never seen a HK taxi where the driver's seatbelt is properly worn. It's always worn too loose with the help of a wide assortment of tools. I've seen the following items used to keep the seatbelt loose for the driver: binder clips, bag sealers (ie, those things that you use to seal an opened bag of chips), folded pieces of cardboard, tape, safety pins.
He's essentially not wearing the seatbelt at all. It's just draped over him. In an accident, it would probably be a choking hazard (if he's not thrown through the windshield).
2. Flash emergency lights when hard braking.
I suppose the driver's intent is to warn the car behind so that it doesn't slam into him. But the logic just doesn't make any sense. If the car behind is going to rear-end you, blinking yellow lights at him won't do anything. Besides, every car already comes with a feature that warns other drivers when brakes are deployed. They're called brake lights. If the driver behind isn't pay attention to your brake lights, they sure are not paying attention to your emergency flashers. How's this for an idea? If you want to minimize the risk of someone rear-ending you, don't hard brake.
3. Flash turn signals when going through bends.
Turn signals are meant to notify other drivers of changes in directions of your vehicle; not changes in directions of the current road. Steering left through a bend is not the same as making a left turn. It's not like you have the option to go any other direction. It's like telling the drivers behind "Hey, the road is bending left. Rather than driving my car off the road and down the cliff, I will be following this bend. I just thought it would be good to let you know that."
4. Flash turn signals midway through a lane change or a turn.
Why bother? This is like calling someone 29 minutes into your appointment to say you will be 30 minutes late. Don't even call. There's no point. You've already pissed someone off, so you might as well just show up.
5. Shift to neutral and apply hand-brake when stopped.
I suppose the driver's intent is to save fuel. This is an ineffective habit and is often used in combination with numerous other fuel-wasting behaviors. For example, hard braking, hard accelerations and tailgating.
6. Fail to shift back to drive and release hand-brake prior to stepping on the gas pedal.
This poor habit just damages and shortens the lifespan of the transmission and brakes. If you're going to practice the ineffective habit of shifting to neutral and applying the hand-brake, the least you can do is to be able to properly reverse your actions to get the car moving again.
On a related note, I've seen some drivers shift to park as an alternative to shifting to neutral and applying the hand-brake. They do so even in stop-and-go traffic. I can't even imagine the type of wear-and-tear their transmission goes through.
7. Occupy two lanes for the longest possible time while changing lanes in stop-and-go traffic.
I can't even rationalize why people do this. When changing lanes in slow moving traffic, either do it by steering hard into direction of the lane you want to switch to or don't do it. Why do people think it's ok to merge gradually into a lane when it's obvious that they're just holding up both lanes unnecessarily.
8. Line plush dolls across rear window.
I don't mean one or two plush dolls. I'm talking about the entire cast of Winnie the Pooh and all the variations of Disney's Stitch. If it's a woman behind the wheel, this gives off the impression that she's childish. If it's a man, it makes him look like a sissy. In terms of safety, it compromises the driver's ability to see through the rear window.
9. Line figurines across the dashboard, often right on top of where the airbags deploy on the front passenger side.
Again, I don't mean just one or two of those bobble-head things. I'm talking about every single McDonald's Happy Meal toy ever made. First two sentences of #8 applies here as well. In addition, people who do this are just putting themselves at risk. In an accident, I can foresee two deadly possibilities. The driver/passenger may ram their head against the figurines. Or the airbag will deploy which will launch the figurines as high-speed projectiles in random directions at everyone inside the vehicle.
10. Flash the right turn signal when entering a roundabout.
The car is steered to the left direction when entering a roundabout; so why does the driver think flashing the right turn signal is appropriate?
In fact, why flash the turn signals at all? #3 applies here as well. There's only one direction to go. It's not like it's possible for the driver to enter the roundabout going counter-clockwise against the flow of traffic.
Perhaps the driver wants to signal to the cars already in the roundabout? If they flash the left signal, it won't be visible to cars already in the roundabout; so they flash the right signal. But it's not necessary to signal to the cars already in the roundabout! Your car is not invisible. They see your car. They know you're looking to enter the roundabout. It's not like they see your car and think to themselves "Hey, there's a car at the entrance to this roundabout. But he's not flashing his turn signal. I guess he's just parked there enjoying the show."
Obviously, not everyone on the road in HK do these things. But enough of them do (especially taxis) to be enough of an annoyance. Are there any more that I missed?