1) A lack of variety.
Ever since the number of collectible koroks and shrine dungeons was released, I've had this worry. It's all well and good to claim you have a massive world full of content, but if that content is nothing but the same thing repeated ad infinium, you don't have a good game. I'm worried that we'll see a focus on quantity and not quality. Adventuring can only cover so much, especially if there isn't anything else fun to do.
2) World size.
I laugh every time a developer says, "But our world is even bigger than [popular open world game]. It has 200% more trees!" There's a reliance on large open worlds with nothing to fill them, and big open worlds that are pretty to look at, but with little substance after you dig a bit deeper. Look no further for shallow open worlds than any Ubisoft title. Looking for a big world with lots of stupid collectibles, but little original content? I present to you Assassin's Creed, Ghost Rescon, and The Division.
That's the kind of game I'm worried Breath of the Wild will be. If it turns out like, say, The Witcher 3, then all will be well.
3) The narrative.
What with the emphasis being placed on exploration and the ability to find Link's "memories" (which apparently fill us in on what went down before everything went down), I'm worried that the game's narrative will be disconnected from the main game. If all the story bits are memories, then that means they'll be self-contained. That means no open world involvement. The worst mistake Nintendo could make with Breath of the Wild is create a distinction between game and narrative, but that would be a very Ninendo thing to do. They don't exactly have a good track record right now.