I've written a pretty comprehensive answer to this here. A brief summary would be that she combined several technologies and ideas to make her superior to older ships. Her turbine engines made her faster than pre-dreadnoughts, and were far more reliable than the reciprocating engines used by pre-dreadnoughts. Pre-dreadnoughts were armed with a combination of calibres, an armament that was only useful at close range. Dreadnought had an all-big-gun armament, allowing for more effective fire control, and for a heavier broadside. These were supplemented by better range-finders and an early form of director firing. Her armour scheme had also been improved compared to her earlier counterparts. She could use her superior speed to stay out of the effective range of an older battleship, while keeping her target well within hers.

Just a note of pedantry here, she can be referred to as "HMS Dreadnought" or as "the Dreadnought", but not as "the HMS Dreadnought" which is grammatically incorrect.