Phone calls - makes a lot of noise and interrupts me regardless of what I'm doing. I want some people to be able to reach me right away, while I couldn't care less for others. Creating barriers for interruption based on: a) the person calling b) the time of day c) the activity I'm involved in needs to become more accessible and intuitive.
Something I've always wanted is to be able to set up my home phone so that between (say) 9pm and 9am, when people call they get an automated response that says, "The person you are calling doesn't want to be disturbed at this hour. If your call is urgent, press 1 to ring them; otherwise, press 2 to leave a voicemail without ringing them." Technologically, it's nearly trivial.
I would think that one problem right now is that not many people are really thinking like that yet. So amongst the people who do think about that kind of setups, each solution tends to be pretty specific to their own use.
That's why you have on one hand Google Voice which presents some options around what I would call "programatic telephony" but not fully customizable, and on the other, OpenVBX's kind of solutions which are highly customizable but not trivial to setup for "regular" people.
But yeah, it's now trivial to do and I'm sure it'll come with time.