I can see where you are coming from with Ace too, as there have been deaths of much more important secondary characters in other shounen manga, Jiraya in Naruto for example. Would knowing Ace even better make us even sadder because he died? Definitely! But it was still very sad and toucing even though we didn't know him a lot. As for the backstory, Oda couldn't show it before Ace's death because it would feel out of place (it was kinda big to just stick it in the middle of the plot) and the whole ''Luffy i will never die'' speech of Ace would lose its meaning and the tragic irony that it represents. I was still crying over Ace's death, because i sympathized with the things he stood for, he had a very cool design, and their bond was really strong, even though Oda hadn't fleshed out their relationship. I really liked Ace's little fight with Akainu, as well as Ace's epic quote ''this era should be named after WB''. I don't think that Ace acted as an idiot, he just acted on impulse. Try and imagine getting your life saved in the last moment, when you had given up hope, and din't even know if you deserved to be born. Just try and imagine how much grattitude you would have for the person that showed you that your life mattered, and came to save you (WB). Now try and imagine someone ridiculing him in the worst way, right after he has saved you. Ace's action was totally justified! So with that being said, not knowing Ace well enough wasn't one of the major problems in the War, imo.
As i said again, my two main gripes with the War, was the lack of Haki display (huge plothole) and the whole ''Luffy leading WB's attack'' fiasco, which implied that without Luffy, WB wouldn't have even released Ace.