I get the feeling from your posts that you mostly work with picture books. However, if you work at all with MG/YA, I'm curious what the upper limit freak-out length is for you. Where you just say, "No way am I reading something that long!! Don't even send me the first page!" I know that contemporary is usually shorter than fantasy, and I've seen both long and short historical (but they seem mostly shorter than fantasy, at least in kidlit). I'm curious because I see some people say 75K, but bestselling MG/YA novels these days consistently include a huge percentage of longer books (closer to 100K--and this includes first novels).
You're right, middle grade and YA readers are showing a higher tolerance for longer fiction. Which is great.
I will read the beginning of absolutely anything. Neither topic nor word count nor author's background tells me what the writing is like. If the writing is awesome enough, heck, you can get away with This Is All. But if you're writing long, make sure you're appealing to the readers who want long. Some people say all of writing is knowing your audience. I say all of idiocy is generalizations. But remember that part of your audience is the editor who is going to throw your manuscript across the room if more than 20% of your manuscript needs to be cut.