Dragon Age: Origins
1) All the missions that are related to the decision on the Dwarven King.
2) Ridding the Dalish Elves with their wolves problem.
3) Rescuing Arl Eamon by getting the ashes.
So only quests that are given by NPC's that have no role over the main plot can be skipped.
[Edited by HonestGamer, 3/28/2011 11:50:00 PM]
You forgot one that also has to be completed and I can add a mod to one of you listed:
On your Saving the Dalish from Wolves, you actually can save the wolves and the Dalish, or have the Dalish killed by choosing the wolves.
Also to skip side-quests, just ignore them and leave the progress where it is, you can proceed to main quests and you will be able to finish the game with no problems