Has anyone else noticed that when you play as a Knight there is SOOO much more to do in the game then as an Assassin or Theif. Knights could do every single quest in the game basically. But Assassins and Theifs can barely do half. Knights also have more random things to do in towns and such. There is absolutaly no one worth killing, and nothing worth stealing from in the game for an Assassin or a Theif.

There is with a bit of imagination

Make your own quests. If you're an Assassin, pick someone in a town, your designated mark. Watch them, plan what you're going to do. There's nothing more satisfying as an Assassin (IMO) than watching your carefully prepared plan work exactly as intended. Of course, sometimes it goes wrong and you have to think fast. But that's just as fun!

If you're looking for work as an assassin, consider accepting tasking from the Blades. They are trying to track down and eliminate Mythic Dawn sleeper agents that are hounding the potential Champion of Cyrodiil. Of course they will require you to deliver the souls of your victims to them in the appropiate soul gem as proof.

As a thief, you can put your own missions together as well. For example, steal Umbra's sword from her - without killing her. Yes, I have done this many times; it just takes some ingenuity.

As Count Lauriel says, just takes a little imagination.