"The currency thing keeps coming up so let me try to address that as best I can.

High runes in Diablo II became a currency for a few reasons. One (and most importantly) because they were necessary to create some of the most powerful/sought after items in the game post-1.10, two because they were duplicated allowing enough of them to be common enough to become a currency, and three the actual currency (gold) was essentially worthless. Before high runes the currency was Stones of Jordan, again, duping is the main issue here for how they were allowed to become a common currency.

Das Inventar in Diablo 3

If they were only being picked up legitimately there simply wouldn't be enough of them circulating to become a common currency.

Now, just because word "rune" is used for the runes in Diablo III it does not mean:
A. They will be the most sought after items in the game (although they could be)
B. Duping will be an issue allowing rare items to become common
C. There won't be a better and more appropriate currency"

"BUT, despite all this, we still want item trades to be useful. There is nothing wrong with people trading items for other items. It only becomes an issue when the barrier of entry into general trading becomes prohibitive due to secondary currency markets based on illegitimate items. And it won't happen as long as there is a intentional and low-barrier currency (gold), and the game is designed and maintained as to not allow creation of illegitimate items. "