Here's my take on it:
1) No side was a monster. All sides had political angles and a lust for land. It was also largely avoidable. Russia, for instance, had no real reason to get involved other than the fact that the Tsar was a big softy who felt honour bound to intervene (and it destroyed Russia in the process, and helped spark the Revolution)
2) The scale of the fighting, while immense, was no where near the level of WWII.
3) It wasn't a real World War. Sure, the Brits rounded up soldiers from all over the world, and some of the battles did take place in the Ottoman Empire and there were skirmishes in colonial regions of Africa, but apart from that, it was a mostly European war. Sure the Americans got involved at the very end, and their help was useful in forcing the Germans to agree to an end, but the bulk of the fighting was done by Europeans who really, had no reason to fight each other, other than the fact that some ponce in some castle or palace decided "well, I think I'd like a war today!"
4) The Fighting wasn't glorious or particularly skillful. Millions died as a result of the stupidity of the Generals with their 18th Century tactics in a 20th Century conflict. It's hard to really find glory in a battle like the Somme, in which idiot generals told their troops to walk, literally WALK to the German front lines. Well, you can guess what happened to those British soldiers - they were chewed up by Machine Gun fire.
5) At the end of the day, the aims and accomplishments of WWII were far nobler than the aims and accomplishments of WWI. After WWII, The Nazis had been killed, the Jews liberated, China liberated, France Liberated, the Imperial Japanese defeated and a new age had begun for the allies. After WWI had ended, there was nothing to celebrate. Millions had died for pretty much nothing at all, the Tsar had fallen and the Communists were gaining control of Russia, continental Europe was devastated and so many lives were ruined due to crippling injuries. All for pretty much NOTHING. The generals had though WWI would be a cake walk. It proved to be anything but.
6) The soldiers themselves hated the war. They saw nothing good come from it. Unlike WWII, in which soldiers accepted harsh conditions in order to defeat fascism, the soldiers of WWI were treated HORRIBLY by their commanders, forced (at gunpoint) to stage suicidal attacks and live in APPALLING conditions. You have no idea how horrible it was to live in those trenches, with the constant shelling, disease, water, gas attacks and mud everywhere. It was cold, dirty, bloody, and your "battle plan" involved going over the top and attacking the enemy positions head on, which was usually an invitation to get shot with machine gun fire. It was dreadful, pointless and cruel, and the soldiers hated, absolutely HATED the stupid, belligerent, inefficient, careless and proud generals who couldn't give less of a damn about the average soldier. The way the generals treated their men was disgusting - if you were Shell-shocked and couldn't move, they'd KILL YOU for being a coward! They refused to listen to advice or change tactics.