The Dark Brotherhood questline makes actual sense. I was never a big fan of the new guild quest lines. I feel they give you to much trust for someone who just joined, in what I would consider 1 week of in game time.
But the Dark Brotherhood actually makes sense. Consider this, Astrid fancies herself Skyrim's Night Mother when she clearly is not, she abandoned the tenets of the Dark Brotherhood and is running merely a mockery of what the Dark Brotherhood could be.
All the rest of the Dark Brotherhood members are blinded by Astrid's poison and blinded by Astrid's rule. With so many sanctuaries gone, who would the Night Mother choose as a Listener?
The Night Mother needed to choose someone outside of the group. Outside of the DB. Someone who was not so easily fooled by merely talk from a leader who abandoned Sithis and the Night Mother.
Out, all the quest lines, being the Listener so early on makes sense. As for the other guilds not so much.
-Oh you tested my chops by interogating some folks to take their payment and break urns. But right after, BRAND new member and you're going to give him a High End Case that even your top dogs couldn't complete. Yeah, no I wouldn't if I was the leader of the Thieves guild. I don't know you. I don't trust your skills.
-Its only been what a week to about an hour and the Companions reveal their secrets and make you one too. I wouldn't.
-The Psijic orders just happens to think you're the chosen one and has to save the mages guild.