I grew up in a small house in the 90's that was essentially open concept, the small galley kitchen was isolated to one side like a small hallway opposite from the bedroom hallway. But could still see the open space.
Good things:
1. It is nice for entertaining.
2. Easier to keep an eye on children.
3. Flexibility to layout.
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Issues:
1. How often do you really entertain? Not counting family visits, way less often than you think.
2. You will always be on top of other people. Examples see [2a]
3. Less flexibility than you think, tables and chairs still have to go somewhere.
4. Nowadays it is extremely popular to put a half bath in the large space. I don't like listening to people potty, I don't like being listened to while I potty, and potty smells can linger.
5. Do you like to clean? People today don't just drop by like they used to, a shame really, but any time you think people might be coming by, even just to drop off a small item, the whole area needs to be picked up. When you have separate rooms, you can quickly pile things into that area that your visitors wont see, not so with open concept.
6. Surprise parties are now harder to pull off.
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[2a] - Say you have company down, and only want to talk to some of them, like when the women want to talk separate from the men. You want to do homework and have a quiet area. You want to read, your kids want to watch the same episode of Mickey Mouse Club House Again. You want to watch the big game, she wants to watch her "stories". You have two teenagers who like to fight for attention, can't separate them far enough with an open space.