TL;DR: Much More Efficient, No Less Deadly

In 1914, Maneuver Warfare was very much of the late 19th century kind; tactics were more suited to the Franco-Prussian War than what was to follow. Armies were almost completely reliant on horse transport, whereas the armies of 1918 were more motorized, even mechanized, with the British and French making use of large numbers of tanks.

artillery pieces were largely divided into field pieces and siege pieces, with none of the diversification that would take place during the war. Artillery methods in 1914 were antiquated compared to the sophisticated fire plans that were being drawn up by the Armies of 1918. Aircraft were not as numerous, diverse or as widely used in 1914 as they were in 1918.

The lines, columns and bunches in which the infantry of 1918 moved were more widespread and fluid than the skirmisher lines common in 1914; infantry weapons were more sophisticated, hand grenades, rifle grenade launchers, trench mortars, light machine guns, infantry guns, etc. Means of communication on the battle field were getting better, and officers were better able to control the battle.

That said, losses were still high in 1918. Losses from the Spring Offensives (March to July) were c. 688 000 German, c. 418 000 British, and c. 433 000 French. For the Hundreds Days Offensives (August 8th to November 11th), it was c. 785 000 German dead/wounded/missing, and 386 342 taken prisoner. For the British and French it was c. 411 000 and 531 000 respectively.

French losses in 1914 were over 500 000, mostly at the Frontiers, the Marne, the Aisne and the Race to the Sea. German losses were c. 800 000, of 1.4 million, while British losses were c. 80 000, of a force just over a hundred thousand strong (half of these losses were at the 1st Battle of Ypres).

As DuxBelisarius said, the French entering WW1 still had the Franco-Prussian mindset of launching the offensive with lighter and more easily transported artillery support. They also had a very small number of machine guns within their units, believing them to be cumbersome and more defensive, so not what they wanted. The Germans utilized heavier artillery and they loved the thought of sweeping around and encircling the enemy, hence their obsession with the Schlieffen Plan and the mythical "Strong Right Wing". This race to turn the enemy's flank led to the race to the sea and solidified the lines that would become trenches.

WW1 was the war that really showed Europeans that the advantage now laid with the defender, not the attacker, thanks to the machine gun.

The German tactics used in the 1918 Spring Offensives were the beginnings of what would later be the blitzkrieg made famous in WW2. German stormtroopers, closely following their accurate heavy artillery support, would bypass strongpoints and begin wreaking havoc in the rear while the regular infantry and reserves came up behind them to take on the pillboxes and forts the stormtroopers bypassed. This caused chaos in the rear lines of the Allies, leading them to pull units back and the Germans managed to make large gains for the first time since 1914. However (again, as DuxBelisarius noted), the Germans took heavy casualties. The units that took the most casualties were the stromtroopers, Germany's most veteran and best soldiers. About this time, the Americans were entering the war as well in large numbers. The new forces reinvigorated the Allies, and their offensive pitted the large numbers of new Americans combined with veteran French and British troops against German lines whose veterans were either killed, wounded, or exhausted and the relatively inexperienced replacement troops.

In 1914 non- stop trench warfare wasn't a thing until later. In a war with the weapons of all sorts they had, they shouldn't have been fighting open battles as they did like this was still the age of Napoleon. Not long after, they realized being in trenches 24/7 was the way to go.