I disagree. Right now, Naruto as a series is far more interesting than it ever was. When I first saw this show I couldn't stand it at all, just the idea of prepubescent ninjas was hard enough to swallow and Naruto's character didn't make it any easier either. The plot while interesting, wasn't unique at all and could have easily been the plot from numerous other shonen series.
When people say stuff like "Wow, this is the best anime ever!" I think one of two things. Either they haven't really seen any great animes before or they were just really looking for something to get into and Naruto is what caught their eye.
Manga: Right now its as good as it has ever been and it has been consistent through out. Sure we like to nit pick it and complain alot but I think thats just because we got used to such high quality action packed chapters then when things slow down we're disappointed.
Music: Now this is legitimate. While I'm not one that really cares about the music in anime I certainly can appreciate what a difference having great music makes and in Shippuden neither the choice or the placement of music is any good. But I don't think the music is really that significant, if its good it'll certainly make the series better but the series isn't going to suffer simply because of a lack of good music.
Storyline Progression: You talk about alot of things here but mainly I think your problem is that things are taking longer to develop than you would have liked. As far as the manga goes I'll ask you to think of a series that is as long as Naruto that hasn't suffered from the same things you talk about here, either the significant drop off in the number of pages per chapter or the slow development. This is to be expected and chances are no matter how good the series is it'll happen, series like Berserk, Gantz, Bleach, HxH they all suffer from this. The only exception I can think of is One Piece.
As for the anime. The long dragged out fights, the long stare downs, even the ridiculous filler episodes and the amount of filler content crammed into even the actual canon episodes is a creative decision on the studio's part, just like they decided to keep Naruto a "kid show"
The rest of the stuff you mentioned, aside from the character development since Naruto is a truly horrible case even by shonen standards, is just your regular shonen stuff and Naruto is your standard mainstream shonen fare.
I think you're just looking at the glass half empty instead of half full. Even though there is a lot that takes away from it the plot itself has improved and plot is the core of every series. Like I said the manga is just as good as it ever was and the anime is still suffering from filler syndrome.
So I don't think the series is any less interesting but overall, quality wise, Naruto just doesn't have the luster it used to have but honestly Naruto was never that great a series in the first place.