Because gold was easy to get in D2, it had no value. Trading was done using runes. This "mistake" of the developers was in fact a great discovery. Runes could not be farmed, so - nobody was bored farming. If there is a currency in some online game, and that currency is valuable and farmable at the same time (in D2 gold was only farmable), this can lead to boredom. When I played D2, I never did Baal runs more than level 80. After that I would level and MF on my own in level 85 areas and - sometimes killing Mephisto, Andariel. If some people did Baal runs to get items - the boredom level was lower than in D3. Every time the drop was different, perfect items were hard to get, there was more stuff to look after (whites, eth, blues, runes) and there were no broken set or legendary items. It was not something that could be measured in hours like "I have to do 8-9 more hours of this and I am guaranteed an item from the AH even if I don't find anything useful for anyone". This is guaranteed to lead to boredom.