What Makes a Good Thesis?
So what should you look for when choosing a thesis topic?
You and your advisor find the topic interesting
While it's certainly possible to finish your degree if this isn't true, it will be a lot more difficult. When you and your advisor are both engaged in the topic, it will be easier to get advice and stay on top of your research. A topic your advisor is opposed to will slow you down greatly.
You are familiar with the field
By the time you choose a thesis topic, you've most likely been in school for a number of years. Even if you returned to graduate school after time in the workforce, you've most likely taken classes before choosing a thesis topic, and in some disciplines you will have passed a research exam. Take advantage of what you've learned: if your research topic is in an area that you're already familiar with, you'll have a much easier time with the literature review as you'll understand the terminology being used. Otherwise, expect to spend a fair amount of time learning the basics of the field before you can begin your research.
The problem has interest (but not too much)
On the one hand, you don't want to choose a problem that nobody cares about; if you do, solving it may not be judged to be a significant contribution. On the other hand, if many people are working on the problem, it is likely that either the problem is not solvable or someone else will solve it before you do. The best topic will be in an area that people in your field are interested in, but where you have reason to believe you have a new approach that will be successful.