As mentioned earlier, whichever trap door slide I did at Rulantica was probably the last time I got nervous about a ride. I'm not that keen on speed slides anyway - one splash to the face and you're effectively blind until the bottom, where you'll probably get another with a noseful to boot. Start that experience by getting into a box where the floor gives way? No thanks.
Other than that I get a bit twitchy about drop towers, but I always do them and actually tend to find the less impressively engineered ones to be more thrilling (within reason). I'm itching to ride Donjon de l'Extreme again - ridiculously tall, spindly lattice structure, exposed seating, late brakes and a very obvious feeling of the mechanisms preparing to drop you. Atmosfear meanwhile is similarly tall, but feels almost reassuringly over-engineered.
With coasters or anything else it's basically the opposite - I'm fine unless a ride gives me reason to wonder if the engineering know-how involved in building it was only just sufficient for it to work. The GIB at Parque Warner is pretty alarming to experience - it's amazing, but refined it is not. Shockwave at Brean (Pinfari looper) was outright horrifying, with minimal OTSRs and wheelsets whose design looked to have been directly lifted from a big apple and scaled up a bit.
The most recent coaster to worry me was Kärnan, which seemed like a combination of multiple concerning factors - huge speeds, Gerstlauer comfort, an enormous vertical lift with those unnerving tilted back seats, and absolutely nothing to brace yourself against when things start getting really ridiculous!