Military strategy had yet to fully understand how to use recent technological advances (particularly, the machinegun, heavy artillery, and the submarine). As a consequence, old strategies turned out to be completely useless (or, worse, massive failures). The manner in which most of the fighting in WW1 occurred reflects military leadership slowly groping about to fully understand how these new technologies worked, and the implications for strategy and tactics. That is, how to effectively use these new weapons, and, on the other side, how to effectively counter their use.
The sad fact is that military leadership on both sides during WW1 was sadly lacking in intelligence and the ability to grasp that these new technologies had radically changed warfare, and that a corresponding whole new paradigm of tactics was required. Their stupidity cost Europeans 4 long years of war, and 10 million deaths. In 1915, new technology had once again created weapons that were more powerful and deadly than the old ones. For one thing, in April 1915, Germany began using poison gas as a weapon. The gas burned lungs & blinded eyes. Soon, both sides used gases against each other. Germany continued developing deadly gases. Actually, the aforementioned tanks, airplanes, and chemicals were generally the ONLY significant technologies invented DURING the First World War. And, overall, none proved to be a significant factor.
Chemical warfare, while horrible, proved to be unwieldy and unpredictable, and relatively easy to counter. After some limited successes against unprepared opponents, the use of chemical gas had very little tactical benefit; the recognition of this limited utility is that after widespread use in 1915 - except for the short-lived effects of the introduction of mustard gas in 1917 - gas was abandoned by both sides as an effective tactic.
Tanks likewise had very limited impact, though for different reasons. They were not available in sufficient numbers to make a difference when first deployed, and the technology was too new - WW1 tanks were too slow, too expensive, had too little armor, and broke down much too fast. Tanks in WW1 were "proof-of-concept", in that they showed a potential to change warfare, but the actual tank available was not up to the task of being useful. Tanks played no real role in the collapse of the German Army in 1918, and had extremely limited successes on the battlefield.
Airplanes were in a similar position as tanks - the technology was really too new and immature for effective combat use. At best, the airplane provided better observation and reconnaissance ability than previously available, but, in a static trench-warfare setting (with the commonly poor European weather), the amount of benefit this provided is easy to overstate. Tactical and strategic bombing was non-existent; the airplane would have to wait for the wars of the 30s and 40s before becoming a useful (and game-changing) weapon.
On the other hand, several technologies which had been developed over the prior several decades first saw widespread use in European-style combat during WW1, and it was these technologies which proved to define the fighting. A short list would include: the practical submarine, machineguns, mass quantities of long-range...