There is a major bug I think people should be aware of.
If you kill all the enemies in an area or harvest plants etc what is supposed to happen is that after about 10 days they respawn.
If you revisit an area between 2 and 9 days after killing them when the bodies have disappeared what is supposed to happen is the respawn timer gets reset so it takes another 10 days to respawn but what actually happens is the respawn timer is killed which means nothing ever respawns there again.
This can break quests and it also means a lot of giants and mammoths never respawn ever.
Someone explains it better in this thread on the bethesda forums. I have tested it myself and what they say in that thread seems to be correct.