In the US and Mexico, avocados are mainly an ingredient for guacamole, or filling for tacos with meats and cheeses.
However, in Brazil and Asia, avocados are generally sugared, and are a dessert dish.
Are there other instances where unspoken “rules” of what is sweet and savory are radically inverted?
Chocolate goes into the very savory mole poblano.
Bread. Most commercial sliced bread in the US is waaaaaaay sweeter than other countries. It makes Vegemite taste odd. I make my own bread now.
I’ve heard quite a few stories of people from outside of North America who are puzzled by the idea of pumpkin pie for dessert.
I’m given to understand that peanut butter is a savory food in many parts of the world (including the U.K.), but it’s a sweet category food in North America. It’s not really sweet per se, but it pairs well with sweet ingredients (chocolate, honey, banana, etc.). I once explained the peanut butter and jam sandwich to a couple of English gals, and they didn’t believe me. Since then I’ve run across this difference of opinion on the internet several times. Some folks claim that American peanut butter is sweetened whereas it isn’t sweetened in other nations, but the stuff sold here in Canada doesn’t taste especially sweet to me and we also consider it a sweet category food.
You are right, it is not really sweet, it is really, really sweet. Like second ingredient is sugar and the third ingredient is another kind of sugar sweet. It is Nutella without the chocolate.
The peanut butter in my (US) cupboard contains peanuts and salt. No sweeteners. And I consider it in the “sweet” class of foods - it goes with sweet things, and I tend to think of it as a sweet food even though it has no sugar in it.
Candy that is spicy is found in Mexico and India – so I’d count that. I’m thinking of things like sugary balls of tamarind “dough” which have a dusting of chili powder. These are usually local-shop-made street market kinds of things, but I’ve seen a few commercial candies like this, too. In the US, we associate spicy with savory, not with sweet…although, come to think of it, we do have “hot tamale” candies and those spicy hard candy spheres (but they’re made with cinnamon, not with chili).
This one is a bit of a generalization, but the concept of a savory pie (and really any combination of pastry and meat) is much more popular in the UK than the US. While the US does have pot pie, that’s pretty much it - if you say “pie,” almost everyone assumes you’re talking about fruit pie. But it doesn’t work in reverse, as the UK has sweet pies.
Don’t laugh at this entry, but grits are good both ways.
I’m sure there’s peanut butter with sugar in it but I’m in the US and mine only has peanuts and a little salt.
I use it to make Thai peanut sauce, or to coat chicken before frying more often than I’d spread it on bread with jelly. I think peanut butter is a “crossover” food that can be either sweet or savory, depending on the cuisine.
True, but when I moved to Michigan, I found that "pasties" are quite common here. Sort of like Cornish pasties, it’s a Michigan thing, except that article might be dated because they are common in south-east Michigan as well.
Oh, and another US/UK one - in the US, pudding is a kind of custard, while in the UK it can mean anything from a type of steamed cake to a rich sausage. That might not be what the OP is looking for, though, because the actual food changes.
ETA: Huh, I’ve never seen Michigan pasties before, chiroptera! They look good . . .
Since you asked for countries’ differences this may not count, but as regional differences in the USA go, barbecue sauce runs the gamut. So does potato salad.
In passing, there’s the old thing about tea in the South: you boil it and them put ice in it; you sweeten it and then put lemon in it. It don’t know what to do.
Yes, they’re just as good in a garbage disposal as they are in a trash can. 
Peanut butter was the first one that came to mind for me, as well. Another one, thought a bit more esoteric, is pierogi and tamales. Most Americans know them only as savory foods, but both are available in both sweet and savory varieties.
Popcorn. Savory in the United States, universally sweetened in South America. I don’t know how it is served in Mexico, but I suspect sweetened popcorn is a Latin American thing.
Is South American sweetened popcorn similar to US kettle corn or caramel corn?
Oh, that’s a good one. I remember going to a movie theatre in Germany (Freiburg, I think), and getting a box of popcorn. Imagine my complete surprise when something I was fully expecting to be salty turned out to be sweet when I first popped it in my mouth.
They spread from the UP. I’m surprised to hear they’re common in SE Michigan; it wasn’t very long ago that outside the UP it was pretty difficult to find them.
Up here, they’re ubiquitous. Churches bake them for fundraisers, most towns have at least one pasty shop. My town has 2 pasty shops, plus a smattering of other places where you can find them (stores, bakeries, etc that don’t specialize in pasties but often have them.) I grew up having pasties for hot lunch at school at least once a week.
Mmmmmm… pasties. Now I want one.