As Rohan notes, we're effectively reduced to watching a small window into a tale that only makes sense if you have read the associated novel. Assuming that you aren't afraid of spoilers, WoWhead's summary details what appears to be the entire contents of the book. It is only with this additional context that the story even begins to make sense - both the motivations of the characters to upend a truce that has been in place since Warcraft III and the details, such as how the Focusing Iris (widely seen by players in the puggable raid finder version of Dragon Soul) ends up involved.
This is a bit disappointing coming from a company that originally made the decision never to spam players with more than 250ish characters of quest text when they could instead be showing players the story in-game. Part of the mystique of the old Alliance Onyxia attunement questline was how the players actually were the heroes who unveiled the black dragon's treachery. Reading the synopsis of the novel, there are numerous places that could have been opportunities for players to participate in a major lore event that does not seem to have needed to happen prior to the expansion launch. Instead, all of these things are reserved for another medium, and players in the actual game are just left to view the aftermath.
Don't get me wrong, I don't always expect my characters to be the most powerful/important characters in the lore of the game. LOTRO somehow manages to make the player characters seem significant even though some NPC noobs named Aragorn, Gandalf, and Legolas (stereotypical Elf Hunter), et al keep stealing all of the best kills. Blizzard just doesn't handle the presence of external story material as well, hasn't done so for a number of years now, but they seem happy with how they're doing it.
(Ironically and cleverly, author Christie Golden has written your Alliance characters - yes, yours personally - into an un-named cameo appearance. Apparently someone failed to tell her that the scenario was 3-player instead of the customary 5, but the idea of having Jaina acknowledge the presence of un-named Alliance allies in the book is a relatively clever tie-in.)
Other general comments:
- Scenario gameplay is a reasonable group-like experience that can be completed by three DPS players, though you can expect to spend more effort on staying alive through use of cooldowns and other class tricks that often get shelved with a real tank and healer. There is also a focus on pulling multiple soloable mobs, such that each player can take their own share if no one player can tank all the mobs at once. As a solo player, I think they nailed the gameplay, in that it's much more like what I actually experience when playing the game.
- I assume there is a limit to how many instances can be up at a time, as the system displays wait times of up to an hour (though I usually wait a fraction of that).
- The loot seems nice enough, though it's unclear what if anything determines what items you will get, how many times you can expect to run the event if you're after specific items, etc. On the downside, it may not be worth paying for gold to gem and enchant gear the week the expansion comes out, but at least you can keep the stuff banked for future transmog/appearance use.
- I didn't get to try the event on my Horde warrior, as he does not possess the required gearscore and I simply don't care enough to farm for this problem. Judging from WoWhead's video, the scenario appears to be as close to mirrored as it could be given that the Horde has to win in preparing to destroy the city and the Alliance has to win in cleaning up the aftermath.