What are the main differences between undergraduate, master's, and doctoral theses?
In a Bachelor or Master thesis, you have to show that you are able to apply the knowledge of your field to solve a typical problem in your field.

In a PhD thesis, you have to show that you are able to extend the knowledge of your field to solve new problems.

The distinction between a Bachelor and a Master thesis may be a bit subtle. Generally I think a Master thesis should show a significantly greater level of independent working. While for a Bachelor thesis your advisor could tell you which methods to use to solve a specific problem, for a Master thesis you could be expected to choose the methods on your own, and maybe adjust existing methods to better fit the problem at hand.

These are the principle differences I see, but of course its also depending on the specific student. A good Bachelor thesis can be better and include more independent work than a bad Master thesis.