This guide hits all of the key differences I go by (the capital R and G; the lowercase a), and a few I hadn’t previously noticed.
The best part of the whole thing, though, is something I discovered in a comment on the swissmiss post: Helvetica, the Shirt. Sweet. Every one of these is bound to piss someone off, albeit for a different reason: the one actually in Helvetica will enrage those who hate Helvetica on principle (it’s overused, it’s not really neutral, it is really neutral, etc. etc.), while the one in Arial will piss off the Helvetica lovers (including myself), if they bother to look closely enough to see the differences.
The one in Cooper Black… well, I don’t know; for me that font will always be associated with Garfield in the early ’80s. I actually like Cooper Black in a cheesy, retro sort of way. But I think the point is probably that Helvetica is so neutral, or at least aspires to be, whereas Cooper Black is the epitome of a font with a personality of its own. (Well, OK, that title probably really belongs to Rosewood, but Cooper is at least a somewhat versatile.)
And then, of course, there’s the one in Comic Sans, which we will never speak of again.