Not able to complet quests

Would you happen to be in a raidgroup? That's the number one cause of quests not being able to get credit for kills, picking up items, etc.

If that's not the case, your UI could be influencing things and a reset is recommended.

To do a full reset UI, you will want to save your addons in another spot if you wish to preserve them.

Then rename/delete or move your WTF, Cache and Interface folders while WoW is shutdown. Reboot your system.

Note, if you are running Vista make sure your permissions are set so it does not undo your changes.

More detailed instructions for different operating systems, including Macs are here.

http://us.blizzard.com/support/article.xml?articleId=18761&searchQuery=reset%20UI&pageNumber=1