The major strength of CPUs is forked decision-trees. If it is not known even which planet one might end up on, or even, without some analysis of price lists from various planets, which planets offer the best rates of return for a given set of skills and state of peace, war, alliance or whatever between planets and the nations on those planets, it is hard to hard-code a fixed set of actions that will work constantly.
I suppose though I am really meaning PC mining as in Personal Computer mining, since the ability to keep historic prices on disk to compute trends and so on is also something GPUs and ASICs are not optimum at.
Daytrading automatically would be an example of the kind of mining I am thinking of.
Basically it is a matter of teaching your computer to make money automatically 24/7.
Once upon a time posting messages worked well, gradually the rules posting had to follow got harder and harder, like instead of only post to no more than 5 newsgroups the same post, it started getting into whether the post involved products or services for sale.
Once upon a time generating web-pages worked. You could generate gigabytes of pages of Yahoo search results on a topic and Yahoo would rate it number one because apparently it thought its own results were the most relevant possible content about the topic.
Once upon a time simply having autosurf programs running 24/7 in browsers worked. Heck that one still works somewhat for some people it just takes a heck of a lot of pageviews to earn, so that the electricity and network-bandwidth cost becomes more than the thing makes unless the pages it is showing to other "surfers" suck the rare actually live human viewers of the occassional page (e.g. people setting up still thus actually seeing some pages other people are showing) into extremely efficient marketing-funnels.
And so on. Mining World of Warcraft gold is really just yet another "how to rake in money automatically" plug-in.
See Total Automation Project, which was known as Money for Dummies until the "xxxxx for dummies" corp complained about it:
http://web.archive.org/web/20040803020712/http://isp.knotwork.com/dummies/See also Makemoney Knotwork: Toward a GNU "make money":
http://web.archive.org/web/20070429184251/http://www.makemoney.knotwork.com/-MarkM-