"...community created mods, including mods that buffed drop rates for single player fans" is true. There were a ton of mods, and they could be used in multiplayer, they just couldn't be used while on Battle.Net. There were a number of B.net-only drops and bosses that non-B.net players had to enable via mods. Luckily, most of these were simple to enable.
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Well, part of that increase could also be due to the improved trade mechanics on stash tabs. I know even I have been tempted to set up a trade tab, but then I remember I'd have to pay attentions to whispers and stop playing the game when someone wants to trade.
EDIT:: I'm not saying it's a huge factor, but it is a variable.