So I was thinking some deep, serious thoughts about Nisemonogatari, as you do. Specifically, I was trying to piece together reasons why the first episode of this season might be considered inferior to last season. Obviously a big part of this lies in the quality of the direction and storyboarding but that's a given, so we'll leave it aside for the purposes of this evaluation.
In the beginning of Bakemonogatari the premise appeared to be 'normal' (yes, he was a vampire, but that whole plot has very little effect on him as a character or the plot) guy runs into an abnormal situation - the 'very light' girl. This encounter takes place at the start of the episode and sets up the story for the preceding two episodes. The next scene is all about Araragi and Hanekawa discussing this girl and then the scene after that is when Araragi has his aggressive encounter with her. Finally they go together to see Meme so that they can begin to resolve the problem.
This all leads to a very focused plot where there's lots for the viewer to discover that they don't already know. We don't know who Araragi is, what he is, who Senjoughara is, what's causing her 'lightness' problem etc. It's all about the interaction with this one key character: Senjoughara. The discoveries that we make in relation to her, her backstory, her interaction with Araragi, her character etc are all fresh and interesting.
The start of Nisemonogatari involves a fun scene between these two characters again. However, we already know who they are and what they are. That's all kind of been done. Then we move onto the sister, who is at least a character that we haven't dwelt on before. Yet, before we can get to her Nadako is on the phone. Ah, but before we even get to Nadako it's snake girl! We already know who these characters are, there's not much to discover about them that we haven't already covered. The only character that's new and relatively unexplored is the sister but the show doesn't devote much time to her. As the focus on this episode is somewhat lacking it's hardly a surprise that some consider it kind of underwhelming.
In short, despite what people might say, this is a show about the characters. Returning to characters that we already know a lot about, (apart from maybe Senjoughara) seems less interesting than learning more about new characters. Or, at the very least, it would seem better to focus on one character in greater depth. There's certainly something to be said for Araragi's more serious discussion with the snail but even that is only one part of a rather erratic episode.
It's interesting though. And I think you're right in some respect. While Bake was very much structured to be a sequence of character pieces, introducing us to the waifus, it did only that. Araragi was nothing more than your typical selfless caricature, saving them. A bit wordier than your usual audience insert, but empty words don't make character.
Nise seems to want to rectify that. Using Bake as a base to build from, it hasn't stopped in its focus on character, its just switched to one that the audience is already familiar with, but never really knew: Araragi. And I feel as if its less effective in that because, (One) With their ovbious audience pandering antics, it's hard to suddenly take any of the waifus seriously as devices and, (Two) You nailed. With isin/shinbo structuring it the way they have, Araragi feels too familiar, though, in reality, he's had no real development at all, to clarify my previous statement. Aside of course, from the fact that he's selfless, and a hotty vampire.
They're trying to go from no development, to a sudden pile of it, and people aren't adjusting well, it seems. Every scene in that first ep, as disjointed as they were, make perfect sense if you look at it that way. If in the next ep he visits the other waifus, and they somehow proclaim that they'll protect/help him, I'll actually be kind of upset that it's wired so obviously.