Warcraft Econ has a very interesting short post up, in which they did a rather simple experiment: They stood motionlessly for 1 hour near some pool in Wintergrasp and fished, and then sold all the fish on the auction house. The result of that was an astounding 500 gold. And I suspect you could further increase that amount by turning those fish into fish feasts if you have cooking skill as well.
With the glyph market on many servers being shot to pieces, fishing might actually be more profitable than making glyphs right now. In fact, as I recently discovered on a level 59 death knight alt, inscription can be extremely profitable if you don't make glyphs. I haven't timed it, but I'd estimate I made a similar 500 gold per hour through the simple process of buying herbs, milling them into pigments, and turning those pigments into inks that I sold on the AH.
So why is it so easy to make lots of gold with activities which require basically no effort at all? Economic theory would suggest that people take the path of least resistance to earn money, and would be all over gold-making opportunities like this, thereby destroying the market for fish and inks. The reason why that doesn't happen in World of Warcraft is that fundamentally boring activities like fishing and milling herbs just aren't fun, and people value fun higher than currency in a virtual world. In WoW gold has the curious property that it suffers from strongly diminishing returns, much more so than money in the real world. The more gold you have, the less an additional 500 gold buys you. Even if you, like me, use flasks, fish feasts, and runescrolls of fortitude in heroics, where they ain't strictly necessary, a single hour of fishing for 500 gold would finance those consumables and repairs for a whole week.
With people valueing fun over gold, economic activity in World of Warcraft is ruled by somewhat different rules than the real world economy. The basic principle of "opportunity cost" still applies, but now that "cost" is calculated in "lost fun", not in time or effort. Some WoW economists get all upset when people say they "farmed something for free", but that notion isn't all that stupid as it appears: If the activity of farming for some reason is not boring, but actually fun to the person doing the farming, his opportunity cost as calculated in "lost fun" is really zero. Thus it doesn't really matter whether lets say killing elementals to farm them for eternals yields you more or less than the 500 gold per hour from fishing. If killing elementals is fun to you, and fishing is boring to you, the elemental farming is simply the better option.
And that is actually a good thing, because it avoids everybody rushing into the most profitable economic activity. Different players find different activities in the game fun. One guy likes to buy and sell goods on the auction house, another likes to make a thousand glyphs, one guy finds fishing relaxing after a stressing day at work, another prefers doing daily quests for gold. It is of absolutely no use to tell the guy who is doing daily quests or farming elementals that his activity isn't the most profitable, and that he should do fishing or AH camping instead. If the former is fun for him, and the latter isn't, the "fun opportunity cost" of doing something boring is simply too great to be viable. The homo economicus is alive and well even in virtual worlds, it is just the concept of what exactly "utility" is to a MMORPG player that is different in the virtual world compared with the real world.