this presentation I watched some time ago is cool http://www.organizedwonder.com/videos/310 although as with arial, I couldn't tell you why helvetica is bad.
The reason Arial is often, arguably inappropriately, slated as a bad font is because it stacks up poorly against other fonts in the situations where it's appropriate. Helvetica is more sterile, balanced, and tasteful — it does most of the things Arial does, but better. The only advantage Arial has over Helvetica is at smaller sizes on the web, where the head of the t, tail of the g, and terminal of a help to make the characters more distinguishable (in theory).
So beyond quality, it's about what's more appropriate. Then of course you've got to set it well, but that's practice.
