Theoretically feature requests are additions or changes to the core Obsidian app, like the editor, pane system, etc.
Given how extensible we want Obsidian to be though, I expect to see more posts under “Plugins” than under “Feature requests”. If a suggestion does not make sense for the vast majority of users, and it does not require special access that the plugin interface probably won’t provide, it should go under “Plugins”.
In reality, we’re all still just figuring this out. So we might be moving things between these two categories.