Fishing, cooking and first aid were three professions I didn't fully "get" when I first started this game. My first toon didn't do either first aid nor cooking for the longest time, but fishing was something I got into at about level 22, when I started playing the Battlegrounds.
Of course, back then there weren't 24%26#215;7 battlegrounds with seconds to entry from the Horde side, so I'd sometimes have 10 or 15 minutes between rounds. I'd sit in Thunder Bluff and fish in the pool on the main bluff, to pass the time.
I joined a cool levelling guild before level 25, and while doing the Wailing Caverns instance with my new guildmates, someone handed out Savory Deviate Delight to everyone in the party, and that's when I learned what silly but neat trick one could perform with that meal. I asked where to get some of that myself, and was told that Deviate Fish could be caught within WC or in the various Oases within The Barrens. The fish would then have to be cooked by a level 85 recipe for Savory Deviate Delight.
Deviate Fish were probably the first fish I fished for within the "oil slick" spots in the water. To my delight, I also caught other random things at times - equipment, potions, and sealed trunks of various qualities. The trunks often contained bolts of cloth and leathers that matched my level of fishing, but at times there were true gems - profession recipes and equipment with low drop rates elsewhere, both which sold quite well to other players in the Auction House. As my fishing level increased and I fished in higher level areas, the cloths and leathers I pulled out could be sold for more than a few gold, too!
Oily Blackmouth are another fish type whose pools I made sure to fish out when I'd run across them in my adventuring. Low level alchemists use quite a bit of it, so it's something that will always sell, and since Fishing is a skill that is advanced through number of fish caught, not level of fish caught/difficulty of fishing area, any kind of fishing is good for advancement, which lets me focus more on the money that can be made by the fishing itself.
As a Hunter, I also found that fishing was a great way to keep my pets fed and happy. In fact, I often found it was too inconvenient to keep a pet that wouldn't eat fish - canines and gorillas, for example - and thus I stuck to cats and turtles past level 40 or so. For my lowbie toons that don't need the fish themselves, I've often sent stacks of cooked fish off to random Hunter toons between level 10 and 15, hoping my donation would help them keep their pets happy for longer.
Today I'm fishing Floating Debris pools and the occasional Sagefish pool when I run across them with my level 15 Priest. I've pulled out 5 bolts of linen and about the same light leather, plus a few bits of equipment I can sell for 50s or so each, and got some Stranglekelp as well, which looks like it'll sell for 50s as well. Nothing major, and I wouldn't want to sit and fish for hours on end to "grind" my fishing level up, but when I do it as I run across the promising pools during my natural course of battle, it's nice to know it pays off and helps me cover my training and equipment upgrade costs!