Sometimes I do want my friends to wake me up in the middle of the night:

"I'm drunk, I need a ride home."

"Can I stay at your house."

"$emergency"

"Hey, I really just need somebody to talk to..."

etc. etc.

I don't want to have to go through my entire contact list, individually setting people who can and can't wake me up at night. There is already a mechanism for this, it's called "human interaction". My friends know who can and can't wake me up; they know this because they're my friends.

By giving them my phone number, I'm trusting them to respect that, and only wake me up if they need to.

I've been wishing there was a layer between my phone and me that said, effectively, "You've reached David's phone - if this is an emergency or he's expecting your call, press 1; otherwise, press 2 to set up an appointment for a later call, or 3 leave a voicemail message." Where 2 would talk to my calendar and ideally their calendar and figure out a time that works for both of us without my involvement. Sure, people could just mash 1, but people always have an option to be rude when there's interpersonal interaction going on, and at least this way they have a way not to be rude while still reaching out to me by phone, and a way to actually get through to me in an emergency.

Mr. Number (for Android) comes somewhat close. You at least have the option of determining who can get through to you.

I'd prefer something more configurable as well -- priority list of who can reach me at any time, during daylight hours, when I'm generally open to talking, and never.

As well as a way for those with at least some level of trust (and, say, emergency services) to reach out to me directly.

Combination of Mr. Number and a pretty acerbic voicemail message ("Hi, you've reached me, don't leave a message, email me instead, if you're a good friend, feel free to text") low-techs this solution for me.

with gvoice, people can leave a voicemail that gets transcripted (vaguely) and texted to me. I glance at the text and decide if I want to call them back or not.

I love that capability, but it's not quite everything I want - I do want a way for people to loudly interrupt me in the case of a genuine emergency, but I want them to have to reaffirm that most of the time. The hands-off scheduling would be nice, too. But for what it is, it's rad, to be sure.

There are a lot of implicit social expectations built into different messaging systems.

I have friends who I regularly email, SMS, and IM with. We've never explicitly discussed this, but it's pretty much assumed that sending an SMS means you want a response ASAP, an IM expects a response within a few hours, and an email can wait a day or two. Even though all three are ostensibly messaging systems, there is a social understanding that different messaging mediums have different purposes; simply filtering based on sender doesn't convey enough contextual information to make a meaningful decision.

It's much easier and humane to accomplish this with multiple applets with different characteristics.