Most important decisions: Sided with mages, Bhelen rules, Alistair and Anora rule, Warden alive, Old god baby with Morrigan, The Architect alive, Killed the Arishok, Sided with Mages in Kirkwall, Executed Anders

Here is the list of important decisions made in previous games:

Dragon Age 1

-Hero

Hero Personality: Chaotic Good
Gender: Male
Race: Human
Class: Warrior
Origin: Human Noble
Hero's fate: Alive and well

-Companions

Romanced: Nobody
Dog's fate: Yes
Loghain's fate: Alive as a Grey Warden
Sten's imprisonment: Persuaded the Revered Mother
Sten recruited: Yes
Sten's sword: Returned to Sten
Oghren recruited: Yes
Zevran recruited: Yes
Zevran's fate: Alive and well
Wynne recruited: Yes
Wynne fate: Alive and well
Alistar's fate: King of Ferelden
Warden & Alistars romance: No
Leliana fate: Alive and Well
Morrigan & the baby: Old God baby with the Warden
Flemeths grimoire: Acquired by Defeating Flemeth

-Prologue

Mabari hound fate: Cured
Prisoner's fate: Killed

-Nature of the Beast

Elves & werewolfs: Brokered Peace
Cammen & Gheyna fate: Together
Halla's fate: Saved
Athras' wife: Didn't tell Athras about his wife's fate
Ironbark: Brought to Varathorn
Deygan's fate: Returned to Dalish camp

-Paragon of her kind

Anvil of the Void status: Defeated Branka
Orzammar ruller: Bhelen
Dagna's fate: Left to Study
Mardy's enconter: Did not encounter
Mardy's son: No son
Rucks fate: Killed Ruck, Said he died heroically
Legion of the dead: Proved connection with the noble house
Shaperate tome: Returned tome to Shaperate
Chantry's status: Did not help create it
Zerlinda's family: Reconciled
Rogek's lyrium: Did not complete deal
Orta's & the assembly: Helped join the Assembly

-Broken Circle

Mages or Templars: Mages
First enchater Irving: Alive 
Cullen's request: Did not agree to his request

-Denerim

Ser Landry's fate: Alive
Oswyn's whereabouts: Told Bann Sighard about Oswyn
Sister Justine scroll: Brought scroll to Justine
Slim Couldry's crime wave: Completed
Pearl status: Helped clear customers out of Pearl
Crimson Oars fate: Handled
Irminric's ring: Gave it
Begger's amulet: Returned
Master Ignacio fate: Completed his missions
Alistar & Goldanna: Helped Alistair find Goldanna
Marjolaine fate: Sent her away

-The Arl of Redcliffe

Villige of Redcliffe: Helped fight
Villige of Redcliffe preperations: Helped prepare
Bevin & hes sword: Freed Bevin and returned sword
Owen's daughter fate: Helped escape
Islode fate: Alive
Conor's fate: Alive and not possessed
Bella's fate: Left to start a brewery

-The urn of sacred ashes

Urn's status: Not poisoned

-The battle of denerim

Who killed the Archdemon: The Warden

-Awakening Expansion

Architects fate: Allowed to live
Nathaniel fate: Alive and Well (recruited)
Oghren & Felsi's fate: Reunited
Keep & Amaranthine status: Both protected

-Witch Hunt DLC

Wardens Decision: Warden did not go through eluvian

-Wardens keep DLC

Power of Blood: Gained the power of blood
Sophia & Avernus fate: Avernus dead, Sophia alive

-Stone Prisoner

Shale's fate: Alive and well
Mathias and Amelia fate: Both alive and not possessed
Shale Recruited: Yes

Dragon Age 2

-Hero

Hero Personality: Humorous
Gender: Male
Class: Mage

-Companions

Romanced: Isabela
Hawke & Varric: Friends
Bethany fate: Died in Lothering
Bethany Final battle: Did not live for final battle
Carver's fate: Templar
Bartrand's fate: Killed
Varric hauting: Helped discover the cause of haunting
Varric & red lyrium: Did not keep the idol
Fenris' fate: Alive and well
Isabela recruited: Yes
Isabela's choice: Returned to Hawke
Isabela & Arishok: Didn't give Isabela to Arishok
Merrill's fate: Stayed with Hawke
Merrill's choice: Alive and well
Merrill's eluvian: Destroyed
Merrill's clan: Not killed
Ander's fate: Executed
Ander's actions at Chantry: Didn't approve of his actions
Aveline's romance: Married Donnic
Aveline's choice: Stayed with Hawke
Sebastian recruited: Yes
Tallis angry: Didn't make her angry
Kissed tallis: Didn't kiss

-Prologue

Hawk joined: Smugglers

-Act 1

Hawke & templars: Convinced Templars to leave
Martin's cargo: Returned to Martin
Kelder's fate: Killed
Danzig's fate: Killed
Ginni's fate: Alive and Well
Saemus returned: Didn't return
Ghyslain's wife: Told of his wife's death
Missing miners mystery: Solved
Ser Thrask blackmail: Did not blackmail
Idunna's fate: Killed
Feynriel fate: Sent to Dalish
Keran's fate: Reinstated with Templars
Ketojan's fate: Defeated
Karra's fate: Alive and well

-Act 2

Gescard fate: Killed
Varnel & qunari: Didn't side with Varnel
Hawk & Petrice: Did not side with Patrice
Javaris fate: Alive and well
Encontered Yevhen: Did not encounter
Raiders on the cliff: Discovered looter
Feynriel's fate: Freed and left for Tevinter
Arishok fate: Killed

-Act 3

Nuncio's Request: Offered to Help
Zevran's fate: Let him go
Nathaniel's fate: Rescued
Chanrade & Gamlen: Reunited
Conspirators handed over: Handed to Orsino
Emile's fate: Returned to Circle
Keran's fate: Gave Keran to Templars
Bone Pit dragon: Killed
Sided with: Mages

-Legacy DLC

Sided with: Larius
Malcolm's will: Found