I like Anton's comment on the question:
The problem is that any time a number is associated with a user, the user feels he must make that number as big as possible. Perhaps we need a high-rep role model with a low accept rate to put people at ease. I see that litb is at 50%.
Perhaps acceptance rate should only be shown for users with lower than a particular rep threshold, since it's mainly a tool for distinguishing question pumps from team players.
That said, I like the OP's suggestion too - acceptance rate should only take into account questions with upvoted answers. Questions with no satisfactory answers should not compel the poster to accept one of them. Edit: wow, I can't believe I said that. Unanswered question -> unanswerable question == potentially bad question. Either work harder to make the question answerable, or if the OP found a workaround or some sort of solution himself, document it as an answer and accept that answer.
However, one of my questions on SO is sitting with no answers accepted, and it doesn't bother me at all. I see my acceptance rate as less significant than my answer:question ratio (112:4) or my overall reputation (~3500).
PS. I'm going to repeat a comment by Andrew Coleson from elsewhere in this question, as I think it's excellent and deserves to be highlighted:
If you want to enforce 70% to be "excellent" (nobody will see it this way intuitively), display a label like "Excellent" instead of the percentage (70%). Basically, put the percentages in the FAQ and just gloss over the details with a tagline of "[Excellent|Good|Fair|Poor] acceptance rate".