Belated newcomer to this obsessive fandom. I'd like to say:
I think the fact that humans can transform into Titans is the core of the story. The appearance of the Colossal and Armoured titans was the event that kick-started the plot. Without them, humanity would have gone on growing cosy inside their walls. I think the connection, whatever it is, between humans and Titans is going to be central to whatever message the author has in mind. Without Eren and (possibly) Ymir on their side, humanity would have no hope against the Titans; they needed Eren to even the odds just a little bit.
This is one of the few manga where I haven't found it completely implausible that kids would be sent out to do the fighting. I don't think grown adults would find it easy to learn the 3DM. Lightness, speed and agility are the most important attributes for a soldier fighting against the Titans. Oh, and insane courage. In fact, I'm surprised they don't start training kids even younger, say from five, so that flying would be as natural to them as walking.
I don't think it's any accident that SNK has so many wonderful women characters. Isayama understands the simple rule for creating good female characters: as GRR Martin said, "women are people, too." Isayama's female characters are good characters who happen to be female. That's all.