1. I won't focus on a bunch of Siri items, but one example that always bugs me: I cannot ask Siri to give me directions to my next meeting. The latest OS introduces an answer for the first time, though. It tells me to open the calendar app on my Apple watch, and tap on the meeting, and tap the address. (I don't have an Apple watch.)
2. Mail.app on iOS does not have a "share sheet." This makes it impossible to "do" anything with an email message, like send it to a todo app. (The same problem exists with messages in Messages.app)
3. It is impossible to share a contact card from Messages.app (both iOS and MacOS). You have to leave messages, go to contacts and select the contact to share. Contacts should be one of the apps that shows up in the "+" list like photos, camera, cash, and plenty third party apps.
4. You still have to set the default system mail app in MacOS as a setting in the Mail.app, instead of in system settings. Last I checked, I'm pretty sure you couldn't do this, without first setting up an account in the Mail.app. Infuriating.