I think the button shouldn't appear if the user doesn't have the access to do it (eg. not an admin, has view rights only, etc).

However, if the user can and they expect to do it, but is in a mode/state which doesn't allow them to do it, greyed out buttons absolutely.

In some cases, greyed out buttons should be clickable (eg. forms) and that should tell the user what is wrong. Looking at you, billion dollar airline, where I couldn't complete my checkin due to this annoyance.

Also, what happened to hover messages and little (?) marks? Every software behaves like it expects me to know what to do from Day 1.

I agree for admin stuff, but certain lack of permissions hiding stuff is annoying. It's nice for things to be disabled and have a hover text explaining why it's disabled. Also, yes, (?) and hover text in general is severely underused in popular software.

I wish every button or menu option that has a shortcut would show me its shortcut on hover or in-place so I didn't have to find it in the manual or buried in the settings menu.

What if the user is an admin but isn't logged in as one, and that's why he can't press the button?

> Also, what happened to hover messages and little (?) marks? Every software behaves like it expects me to know what to do from Day 1.

I truly hate fast-food kiosks for this reason: most expect me to be familiar with their offerings and the menu system of that kios. I relatively rarely visit such places and I was better off with a human asking me 1-3 short questions and paying. I'm not paid to know this shit, I'm the customer. Guess I'm just too old, younger people love this crap for some reason.