HA isn't actually hard, but it does require some forethought and chosing some technologies which aren't neccesarily cool.
However, I would submit that building HA capabilities into a cat photo sharing or microblogging site is sortof overkill, most people don't need it. Just take the hit and move on, people are getting more and more used to sites being down/failing and just retry later. As much as I hate to say it, I do think its fairly accurate.
You pose an interesting question that I want to see answered: do users of B2C sites actually care when the site is down? Does it decrease MAU? Your intuition is that it does not, but I'd love to see some data.
A really rough guide. Rough enough that I thought of each of these points last night at 1 am while extremely groggy and trying to bring back the 7 or so instances we lost in the outage.
I guess it is a good play to get traffic to your site.