So, my main character in World of Warcraft is a blood elf hunter. Blood elves are the elves on the side of the Horde, and the Horde is the side in the game whose architecture features lots of spikes and skulls. In true Po-Mo fashion, however, the Horde has this whole Klingon stoic virtue thing going on, while the other side, the Alliance, composed of all the standard fantasy "good guy" races, is full of the evils of everything from feudalism to capitalism. It's best to disengage your political brain from the proceedings.
Anyway, in addition to your character's main "class" (your usual fighter-wizard-cleric-thief tropes are all present and accounted for) you can also learn two side "professions": everything from tailoring to blacksmithing to herbalism to the inscription of magic scrolls can be learned as a side profession. You can gather or buy raw materials and increase your ability to make ever more complex and valuable things which you can sell in an auction house in-game for imaginary money.
For my first WoW character I decided to be pragmatic and picked skinning and leatherworking. That way as I encountered various critters as my character progressed through the game world, I could skin them and then make salable belts and boots and armor bits from the hides. This seemed clever. The problem is that I forgot to keep up with my side professions as I adventured my way through the game, and wound up with a potent level 90 (that's good, btw) hunter who had a hard time gutting a squirrel and making a coin purse from the skin.
The solution? A thing called "power leveling". In my spare time I journeyed through lower-level areas of the game where I had already been and did all the industrious little chores I should have been doing all along, except on the industrial scale made possible by the disparity between my character's level and the low-level surroundings.
Many small groups of lower level characters saw a figure like a Molly Hatchet album cover swoop out of the sky riding a skeletal dragon, hop off and mow down a veldt's worth of wildlife in quantities that would make a party of railroad buffalo skinners blush, set to skinning it all, and then swoop back off into the sky again without so much as waving hello.
So, I'm out doing this in a part of the map that is styled like some fantasy-world version of Egypt, following the forested area along a river through the desert and killing & skinning every gazelle and hexapod crocodile I run across. And I cut loose at a croc with an attack called "glaive toss", where your hunter boomerangs a pair of big spinny blades at the target. The blades arc out, intersect at the croc which flops belly-up, and return to my character's hands. I run over, loot and skin the dead croc, and as it fades out, I see this...
Now, I've been going through this place like a platoon of seal-clubbers run amok, damaging the ecosystem with a single-minded intensity that you'd need to wreck a dozen oil-tankers to duplicate, but I suddenly felt bad about accidentally "killing" the little imaginary frog-shaped blob of pixels on my screen. Bad enough that I snapped a screen shot...
And that's when the game designers can pat themselves on the back, because they managed to get the player good and immersed there. Good job, Blizzard. I haven't had that "in the game" feeling since I tipped my head to the side to look under a stall door on my screen while rooting terrorists out of a bathroom in a missile silo in Rogue Spear all those years ago.