People are supposed to think bisexuals don't exist
, that they're just confused people in transition to gay or straight. I've only met one person who actually believes this, however. It isn't a very threatening misconception—it's easy to point out someone who has "practiced" bisexuality for a very long time.
In my experience, the most common misconception is that all bisexuals feel attraction the same way. Bisexuals are supposed to be gender-blind, attracted to both sexes equally as if there were no distinction between them. Although there are some bisexuals who feel this way, there are many more to whom this does not apply.
The Kinsey scale is a good first pass at quantifying and categorizing bisexuals. Trying to sum up a lifetime of attraction, fantasy, activity and self-definition with a single number is obviously going to be limited.
The rich, deep description of someone's bisexuality comes from asking that person about it. You'll meet one person who finds that each sex has its own attractions (as I do), another who is gender-blind, a third who describes himself as straight and mostly prefers women but can't resist a little oral sex with a man once in a while.
There's simply no substitute for just asking a person. Of course, this defies easy categorization. And isn't that exactly what bisexuality is all about?