Along with Blizzard's 3.1 release for World of Warcraft, there were changes that effect how one fishes in Azeroth and Outland. Where before, there were both a minimum and a no-get-away level for each zone. After the patch, one can fish any where at any level. The catch is, one will catch more junk if he is in an zone that he is too low to fish in. Although, he will still gain fishing level as if he was fishing in a zone that he properly experienced for.
So the questions are, if one is fishing in a zone that he meets and exceeds the required minimum fishing level for, whether it be with or without fishing gear and bait, does he have a chance to fish up junk still, let a fish get away, and fish up the better catch at a higher rate for the zone? Along with those three questions, does the bobber have a higher chance to bob earlier in the fishing cast too?
Note: Please keep in mind that there are four questions with in this one Gaming SE question? Since I already know how to fish, please do not post links to fishing tutorials. These are questions that I have always ask myself time after time but was never able to find answers. I can't be the only one who has thought about these same questions. Maybe I am though?
Best Answer
What used to be the minimum required skill to fish in any given area is now the minimum required skill to not catch any junk. Once you meet or exceed this minimum for a zone, whether natively, or through the use of equipment, buffs and consumables, you will no longer catch ANY junk. If you are below this skill level, the chance of catching junk is proportional to just how far below it you are. The further below the threshold, the more junk you'll catch.
The exact formula for the chance of catching a fish when your skill is below the threshold is ((current skill/threshold skill)^2).
For example, the Dalaran Fountain has a no-junk skill of 525. As we can see in @GreatBigFish's answer, this junk threshold is above the (unbuffed) WOTLK maximum skill of 450. Fishing at max skill in Dalaran with no lure would yield a 'fish' roughly 73% of the time. The other 27% of the time, you will catch one of the 13 'junk' items found around the world, all of which vendor for mere copper. To never catch junk in Dalaran, you'd need to use a +75 lure or better at 450 skill.
There is one notable exception to this rule: You will never catch junk in a pool. Fishing from pools gives a 100% catch rate on whatever type of fish spawns in that pool. This has caused many to claim that actively leveling fishing skill is now pointless because it provides little practical benefit - fishing from pools is always more efficient anyway.
For more information, El's Anglin' provides a tremendous resource on the subject of fishing.