When it comes to roller coasters, one major factor is fear. The fear is generated using a multitude of methods. One method is using heights to invoke the fear of falling to your death. In addition, the roller coaster might be made of less stable material such as wood. Some parts may even "appear" broken, rusted and even shake to a ridiculous degree.
Next would be coaster designs such as lots of random support bars or decorations that might rip your extended limbs off or decapitate you if you're slightly too tall for the ride.
Once you've dealt with those fears you'll only be left with the speed of the coaster and the track design. Sometimes speed can be a blessing in disguise you'll be less likely to focus on the aforementioned fear factors.
Some basic track designs that are feared include things such as loopings, tight turns, and twists. Of the 3 mentioned, loopings can actually be the most intimidating, however they're relatively peaceful during the ride. Not much harder than just a drop.
Tight turns on the other hand threaten to throw you off or knock you into the cart, which in my opinion is just a bad time.
What you should really be scared of is things like twists (where the track is just twisted). The main reason being momentum. During a looping, the momentum is your friend and you'll feel safe. During quick turns, the momentum will try to throw you off which isn't nice. During twists however, the momentum will try to yeet you off continuously to an extreme degree, threatening to kill you. They're not your friend.
Just as friendly advice, the most likely time for a ride to malfunction is right after it starts (oftentimes due to human error). Avoiding being amongst the first few groups of people riding any coaster on any day will minimize the risks you'll face significantly. Just think of it as letting others test the waters.