Maintenance for the week of December 23:
• NA megaservers for maintenance – December 23, 4:00AM EST (9:00 UTC) - 9:00AM EST (14:00 UTC)
• EU megaservers for maintenance – December 23, 9:00 UTC (4:00AM EST) - 14:00 UTC (9:00AM EST)
There is only one quest in the entire game that I know of that has any lasting effect on your character and that's the final quest of the Mage's Guild questline. There are other quests that have minor effects on the game world such as the vampire quest in Coldharbour, but none of them have any effect on future quests. You can't be locked out of later quests by making the "wrong" decision. There are no wrong decisions outside of roleplay headcanon.
Like redspecter said, not many quests change the outcome of the game. There are a couple of quests in which the choice you make changes which quest you do after it, like the one in Starved Plain in Stonefalls and the Narsis quest chain