I felt the same way about a lot of the things you mentioned (hades again, the all-powerful artifact you need to claim to beat the last boss, the useless weapons/items, etc), and I never really thought about the story that way, but I agree, it does seem like they dragged out the ending like that. It could have been handled a lot more gracefully, but I'm still happy they made a third game. I'm not very far in it yet, but from what I've seen so far, I'm pretty impressed, if not by the visuals alone. It still has everything I liked about the series, and the scale and some of the battles in the game are downright impressive. Had God of War 2 ended with you killing Zeus, I probably would have been left begging for another 7-9 hours to stab things in the face.
Even if it makes the story feel dragged out, I think we all still wanted to see this game. Brutal gameplay mixed in with glorious HD and a scale unlike anything I've seen in a game before. The second game will probably be my favorite when all is said and done, but GoW 3 is definitely a solid addition to the series, it's still fun to play, and the impressive visuals and boss fights give us anything we were missing or could have wanted from the series.
Now, my only real problem is finding the god damn eyes/feathers/horns. In the first game it was pretty much impossible to pass by a chest and not realize it. But with each game they made it harder and harder, and it's to the point now where you can pass by some chests accidentally and not be able to go back to get it. WTF is that nonsense.