If you are looking for a very long, very entertaining and very clever game, this is it!
1) The main story line is very good, you travel all around the very large map finding clues, weapons, people and slaying mythical creatures, (think World of warcraft but with better graphics and less repetitive missions, or Fable but with a bit more grown up story line)
2)There are hundreds of side missions, these can be played at any time during the game, these can be found by talking to people and by joining one or more of the guilds (I recommend joining either the assassins guild or the mages guild first as I found these missions to be the most entertaining).
As well as all of the missions just exploring the map is entertaining because you have no idea what you are going to find around the next corner.
3)Each character you talk to has there own individually personality and traits, each fight is different, each weapon and spell is different and cause different effects on the unlucky person or creature that happen to be in you way.
If you like Fallout 3, you will love this; what it lacks in guns and explosions it makes up for in a truly involving gaming environment, you will not be able to put this game down for weeks, I have had this game for about two years now and I am still discovering new map locations and missions, the game really does go on for ever, unlike the rather annoying ending to Fallout 3!!!