The key to good UI design is not simplicity
I wrote a comment (http://news.ycombinator.com/item?id=2044399) in regard to simple CMS systems (and why restaurant websites suck). The key to remember is that the computer illiterate population (e.g. small businesses) have such a problem with technology, that commonly perceived "simple systems" are still too advanced. "As simple as email" is an oxymoron to the small business owner. Email wasn't ever simple, it was a huge pain to figure out! It's just something they suffered through because they needed it.
So the next time you design something for a small business owner (or similar market), don't design for simplicity because it's a nirvana you will never achieve. Design for familiarity with something your target market already knows.
One case in point is Posterous. Tumblr has a pretty simple interface, but it isn't familiar to what we already knew. Posterous used email, which was familiar, not simple.