It seems to have become an accepted fact that the Zelda formula is getting stale. Judging from the sheer number of people who say this, anyway.
I'm not sure I believe this. I don't think the actual formula is the problem so much as the window dressing on that formula, and the designers failure to live up to that formula.

With that in mind, let's look at what the actual formula is:
An overworld map that you can only access a small portion of. Acquisition of items allows you to reach more areas of this map, which allows you to reach dungeons.
The dungeons contain new items; the second half of each dungeon largely revolves around learning to use the new item, while first half revolves around using all the items acquired before that dungeon together.
In addition to opening a new area of the overworld map, the new item also allows your to reach previously inaccessible crannies of the earlier areas.

That's all there is to it, really. This is the heart of it, and why everyone agrees Okami is Zelda-like, since it follows the exact same formula.

I think frustration comes from lack of innovation in the application of that formula, and in poor execution of it.
Items that are only useful in that one dungeon, and almost completely forgotten afterwards. The same items in every game with no real variation. Lack of decent reward for clever item usage - I know one of my biggest problems with Twilight Princess was the way they would give you useless rupees for successfully finding a hidden area that involved several items to get to, but would have Pieces of Heart randomly sitting in trees. Backwards!

But I think the biggest problem is the window dressing. Every game is basically the same. Sometimes they add lots of sailing or something, but essentially they all boil down to the same story, the same characters (except not, which means they can't even build on the history) and the same genre.

Perhaps it all really comes down to me finding Western medieval fantasy settings insanely boring. Okami has just as many problems as your average Zelda game, but because it has an unusual setting, it feels fresh where Zelda feels stale.

I'd like to see a Zelda Gaiden series.
A steampunk Zelda, a Zelda game in a modern urban setting, a sci-fi Zelda, a 30s Noir Zelda - take the basic formula above, and put it in new settings, with new characters. Keep the gameplay intact, but change the window dressing, allow for much more varied items and stories, different kinds of heroes and villains. The word Zelda in the title brings the built in audience, while the new window dressing brings back people who lost interest in the series a few games back. I'd certainly be a lot more excited.