That's because:

1. blizzard had put some basic guides, open up adventure's guide (ctrl+J) and u can see each bosses mechanics
2. They are playing the PTR server and did some research from there.. usually not much changes from PTR to live


For not so top like us, we usually go to fatboss tv or other top guild's strat and read wowhead and adventure's guide

mechanics is not a puzzle, once you read up and know the mechanics, u need to practice to familiarize with it.. it will become hard if you dont respect certain mechanics even if its the easy one, especially the one that will wipe your entire raid.

fun is a subjective thing. one thing for sure is blizzard needs to cater various level of playerbase from hardcore to casual.. so they just put some basic info on what are the skills and mechanics certain bosses have in an adventure's guide and its up to the players on how to deal with it.. i've seen a guild that only read adventure's guide and go raiding and wipe a few times. to them, that is fun thing to do.

wowhead and mmo-champion is a fanbased website contributed by the players, everytime PTR releases, they will data-mining.. and certain guild that plays on PTR like fatboss will put up simple guide on how to do things, but it still up to the players on how to execute it. Maiden of Vigilance is a good example on no matter how much u read and watch, without practice, u still can wipe the raid.