[mst]

Depends, really.

Internal examples:

If shadowcat's public facing website is down for a day, a few people can't read blog posts and maybe we'll miss out on a potential customer - but our existing customers will be entirely unaffected.

If our ticket tracking system is down for a day, it'll annoy the hell out of the existing customers but we can still get the work done since they all have direct email and IM contact info for people.

On the other hand if our ircd is down for an hour, it's time to panic, because that massively interrupts our ability to co-ordinate our work.

External examples:

If linked in is down for a day, I don't care - anything I do on that can wait until tomorrow.

If duckduckgo is down for a day, I am going to burst into tears because I use it all the time for information I want -now- and going via google is substantially more annoying.

So "anything that matters" is really quite relative.