Prosaic ... jesus, that's a new word for me and I think I like it. There are two qualities that define classic-Rare: personality and attention to detail; the former, images and sounds that fill our brains, while the latter consists of many prosiac elements of the games we love -- what we take for granted. But together they make Rare titles indelible (there's a word for YOU) -- quirky/personality-filled games, developed remarkably well. Do I feel similarly to you? I think I feel similarly in that I want this to be the best game it can possibly be, and that the DKC and BK games occupy a special corner of my heart, but I'm not feeling particularly worried. You mention how they tend to focus on the collectables, progression, and arcade machines of the game in press releases and interviews. I'll discuss them below.
The collectibles and progression of the game area are part how Playtonic are bringing YL up do date, this is why they come up so much. Playtonic recognize that DK64 really overdid collectibles, and seem to be looking to streamline from Banjo-Kazooie. An example of this is how move-specific items are replaced by an energy bar (no eggs, red/yellow feathers, etc), this is replenished by a butterfly collectible that can also be eaten for health. They're actually making collectibles more meaningful, less trivial.
Since the 90s, games as a medium have evolved to provide players with more personalized experiences. Again, Playtonic intends to bring platformers up-to-date in this area, as well. This is reflected in open-ended move choices, expandable world, AND the hub world -- confirmed a navigational puzzle, with doors and corridors like Grunty's Lair, as well some larger and more open areas. It will be very Metroidvania in that progression and discovery of secrets will be partially based on what moves you have unlocked. We have not seen or heard anything about straight-up lock and key elements, but learning moves to progress through the hub and worlds -- definitely. Going further with that -- secrets and Easter-eggs are definitely confirmed. (They want to give fans something they can mull-over for months after the games release.)
Arcade games -- hardly prosaic but certainly not what we've been waiting 15 years to play. However to find a worlds hidden arcade room, 80s arcade vibes and all; all to be greeted by a goggle-eyed Rextro Sixtyfourus? NOW WE'RE TALKING!
You mention how important the art direction and humor are, personally these elements were present in the Kickstarter page, have been present in the little updates and glimpses over the last year, and are realized in the recent E3 2016 Trailer. Looks pretty spot on, and I'm not sure if you have any specific concerns in this area. But there is a now iconic quality to BK's landscapes and it is admittedly difficult to compare these previews to enshrined memories from ones childhood. I will say that Glittergaze Glacier looks stellar, especially with the contrasting glacier and rocky landscapes. Tribal-stack Tropics looks really good up close but it's hard to take in intuitively -- I don't see much for a landmark.
You seem to believe this title won't have the depth of Banjo Kazooie. While you note that it "remains to be seen," you also state their ambitions fall short? Maybe you're referring to the drop in world count from BK's nine to five. I may disagree with you. There is still much we don't know about the games actual scope.