I've heard it explained that "hey" used to be more of an urgent way to get someone's attention, rather than a casual "hello" like it is now, so it sounded rude to some older folks.
I've been told no problem is bad to say to customers, as if saying it meant they usually are a problem, but not so much this time. Since I work in IT support though, every client I speak to is either a problem or has a problem by definition.