Sitting here it's tough to think about how long I've been playing WoW. I remember the five year anniversary, so it's been what - six or seven years since release now? Back in the early days of Vanilla I remember running around without any knowledge whatsoever. Every boss was new and exciting - and also irritating when something unknown would happen. By the time we got to BWL though, there were fight videos and strategies everywhere. Raid leaders would post links to strategies and videos, beg their raiders to at least look at it, and then cry giant tears when idiotPlayer5 says "Oh yeah, I know the fights" suddenly wipes the raid on Vael becuase he didn't run away.
How it will affect me now
In our current raid envrironment Gary Busey (no, not the imitation mage that app'd to our guild, but our Pallatank) does a lot of the raid leading. He has quickly discovered that reading us 34 pages of fight notes, and or trying to make us watch videos, is pretty much a waste of time. Give us bullet points man. You know, like telling us there's a phase three where Nef comes out and shits shadow bolts all over the raid. Information that would have been handy yesterday - know what I mean? The last time this happend I felt like Day 9 in his trumpet episode (hopefully embedded below). Shadow bolts? What the!?!?!
So, this new dungeon journal will give us a quick insight to the boss mechanics, instead of sending JM out on a wild goose chase to check BossKillers, WikiWoWPedaiNow (or whatever the hell it's called these days), or any other potentially unreliable source. If you've ever studied a kill video from say the PTR, only to find out it's changed radically on live you'll know what I mean. Damage is also sometimes WAY off - like Cho'gall's shadow nonsense in P2 doing 50k per person per tick (on a 2 seconds timer). You just kind of stand there going - alright, so 7 heals, 2 DPS and 1 tank? The running joke for us in BC were the stage hands in Karazhan that exploded for fire damage. The Wiki writeup simply said "Fire bomb hits HARDER THAN NEF". Thanks. That gives me something to shoot for (?).
Another big benifit for me, is that this journal will give me something to reference when I start leveling my DK up through the dungeons in Cataclysm. Yeah, I've healed them all a thousand times, but that's different than tanking them. Plus now I generally watch a movie while auto healing. Zzzz. Well, until I get the Frost DK who's dual wielding - and didn't get the memo that Blood is the new tanking tree. Then I drop group and make a snack.
A giant leap for PUG kind
Now this tool isn't going to take the Donald's of the world and turn them into the next Kungen (or whoever is leading the charge from Vodka/Paragon - whoever) overnight. It won't help the idiot that can't avoid the sonic rings on Atramedes, who stands in the fire, or what have you. What it will do is give the PUG raid leader a fighting chance at survival. At the very least they can link abilities in party/raid chat.
This is doubly true in heroics and the LFD dungeon. Players are much more likely to spend 15 seconds looking at a boss in this than they are to go out and research it themselves. It's in game, and it's more convenient than an addon. Of course there will need to be some kind of tutorial or auto-popup to let folks know it's available. Maybe it opens up automatically the first time you enter a dungeon or boss's room. Kind of like the paperclip.
I fail to see a downside
Overall I don't see a huge downside to this feature. Blizzard has done a great job (well, ok - decent job) of integrating popular addons into the game - this just rolls up a little BossKillers + AtlasLoot. It's certainly going to be better than having someone say "when you see the big green beam thing, make sure you do the HokeyPokey". It's certainly not going to let the guild made up of window lickers roll over H-Ragnaros - just because you have a ticket to the dance, and even know all the moves, doesn't mean you've got the rythem to pull it off.