If you think back to the Plaguelands, specifically Scholomance, this is something they've been attempting to do for a long time now, likely inspired by the power of Sapphiron and the accompanying Frost Wyrms. So if they have access to the Frost Wyrms... why would they need more dragons? Seems pretty obvious to me. What they can do with a pile of flying bones is rather limited compared to what they can do with (un)living, fleshy dragons.
As we've seen with the Plague Eruptors in the WotLK bestiary, the Scourge has put some focus on new means of spreading the plague. Lacing oats and candy bars with the plague isn't going to work anymore, the world knows what's up. The direct approach will have to do, and what would work better than giant flying warbeasts spewing forth the plague on whatever it feels like?
The involvement of the actual Dragonflights here is interesting, too. I'm a fan of the Red Dragonflight half of the story mostly because I'm a Red Flight fanboy. Grinding rep with Alexstrasza? Yes please! The Blue half I'm pretty torn about. Specifically this quote here:
"Determined to manipulate all existing magic to serve his own purposes, the blue Dragon Aspect, Malygos, has ordered his flight to maneuver colossal machines into key positions over magical ley line clusters."
If I see even one Mana Forge in Northrend, I will... probably do the quests anyways, but I won't be happy about it, that's for sure! I think the story of Malygos in Northrend will be an interesting one, but I was always under the impression he could simply alter ley lines and the like himself, not needing giant machines to do it. While I have no issues with the 'high fantasy' of The Burning Crusade, Blizzard has stated they'll be returning to more traditional fantasy for Wrath. The Blue Dragonflight building colossal machines isn't exactly traditional fantasy. It should prove to be interesting nonetheless.
We also see the return of the Scarlet Crusade here in the Dragonblight. The remnants have dubbed themselves the Scarlet Onslaught. Most likely they play the role of villains in this zone, but my gut says they're going to be friendly or at least neutral, like we see in Light's Hope Chapel. I'd like for them to be villains, that's what they do best, but you never know. It's possible these are the remains of the expedition that went to Northrend way back(which you can read about in the Armory of the Scarlet Monastery) but again, that's just a guess. If the Crusade does become good guys here, it better be Marjhan and the gang leading the show. It might be a little "lorelol" but it'd be pretty fun. Besides, Marjhan is hot.
Angrathar the Wrath Gate and a necropolis we see in the fly over video, which is most likely Naxxramas, are the last points of interest in this preview. The Horde and the Alliance are apparently gearing up to assault the gate, which leads to Icecrown. A sign of an event similar to the Shattered Sun Offensive in patch 2.4? Maybe! I'm curious if Naxxramas is tied to the gate in any way. Perhaps clearing Naxxramas, plus solo quests against Angrathar leads to opening up an entire new zone? I doubt it, but that would be very interesting. The Dragonblight's placement on the Northrend map seems to place it at low to mid 70s levelling, so end-game content besides dungeons probably won't happen here.
Wow, that was actually pretty long for such a brief preview of a zone! As you can see, there's a lot going on in this zone, which is a good omen for the rest of the expansion. If every zone is as packed full with plotlines as this one is, I'll be pleased.