Quantum Computing (circa 2024) - No one has explained it to me but as an OLD programmer who ages ago studied how programming and digital computation work I have created the following broad model for understanding. 

Initially computers did one instruction at a time in sequence - very linear programming.  Computers got faster and more complex.  They then started things like "caching" - preloading instructions into very fast memory. Then another iteration did pre-instruction functions (prefetch, instruction decoding,..).  Moore's "Law" drove chip capabilities further and further allowing even more potential.  Attention turned from electronics to complex software/electronic advances creating chips and software that could execute more than one instruction at a time. This lead to hyperthreading and multi core chips.

My expectation is that quantum computing/computers involves new super fast, super dense computing technologies requiring new advanced programming techniques. For example doing parallel executions on a massive scale but requiring massive parallel validation of outcomes. If I am correct on the infamous Turing Test provided proof of such possibilities. 

The downside is the scaling factors are also gargantuan (both hardware and software) so I don't expect mass practicality in any near future. 

Scale matters! I point to our human brain.  It's "computational" ability is large but even our brain takes short cuts to run real-time results. This suggests nature understands scaling issues and makes practical choices!