On another thread I saw someone mention as a complaint that the main quest lines are a little too coincidental and don't stem from the game's ultimate protagonist (the archdemon and its blight). Essentially they were saying that it didn't make complete sense that all four of Ferelden's major factions are in a state of disarray on the eve of a major war, and that lessens the main story's cohesiveness.
It got me thinking, and I thought I'd make some obvious points and some very nerdy conjecture that could explain the need for each main questline.
Arl Eamon and the Humans: The easiest to explain, Teyrn Logain has poisoned Arl Eamon as well as attacked noble families in Highever in order to set up his claim to the thrown. As the story unfolds, it becomes clear that Logain never believed King Cailan was fit to rule, and the Battle of Ostagar (which stems from the blight) merely serves as a convenient way to elminate both him and the Grey Wardens without blame. In short, Logain is using the blight to satisfy his own ambitions.
The Circle of Mages: Also easily explained. Anyone who has completed the battle of Ostagar and recruited Wynne, knows that a major contingent of the mages marched in the king's army including some of the senior leadership. Upon returning to their tower, the weakened Circle then faced a coup of blood mages led by Senior Enchanter Uldred, who was willing to bargain with demons in exchange for power. As with Logain, Uldred is an opportunist. He has believed for some time that the Circle was being marginalized within Ferelden politics, and that they should have a more aggressive stance. He sees instability as a chance to seize power unrestricted by the Chantry.
The Dwarves of Orzammar: Here the conjecture starts to take over. It is already clearly explained in the Dwarf Noble Origin that the youngest son of King Endrin has killed or exiled his brothers setting himself up for a bid to the thrown. While it is claimed that Endrin died of "grief" over the tragedy in his family, there is still a possibility that Bhelen was behind it as well. He certainly has motivation, so the question turns to his timing and relation to the blight. This can be explained by consulting the Legion of the Dead, who reveal that while darkspawn on the surface are rare, they are ever-present underground and in constant warfare with the dwarves. In fact, the only time there is a lull in the war is during a blight, when the bulk of the darkspawn horde takes to the surface. And so there is the likelihood that Bhelen discovered a blight was underway, and thus concluded it was the safest time to conduct a coup, when darkspawn were not knocking down the gates as it were.
The Dalish Elves: Here I am really reaching for it. In the Dalish origin story, it is revealed that elves are a migratory people (at least Dalish are) who travel as clans across ancient but familiar routes. When the Warden meets them, Keeper Zathrian's clan is currently in an area they have camped in many times, and where they have especially dangerous enemies. Why are they in such dangerous places at the same time as the blight? Because they are escaping the blight. In Lothering, it is explained that Chasind have been moving north in increasing numbers to escape the blight, and if the Warden is Dalish they will explain that their own clan is traveling north as well. Unfortunately I can't readily explain specifically why they just so happened to be in the vicinity of some of their oldest enemies at just the "right" time, nor why the werewolves picked this occasion to go all the way in their feud.
Anyways I am probably reading into this game waaaaayyy too much.