What are characteristics of a "good" thesis topic? This may be too discipline specific, but I'm going to give it a shot.

I'm still relatively new to grad school (6 months), but I've been told by several older students to keep an eye out for potential thesis/dissertation topics.  I'd like to start a list of potential ideas for myself - I'm sure some will be solved before I start my actual thesis, and others are already solved and I just need to read more. But what are some characteristics of a "good" thesis topic?
I completely agree with ff524's criteria, but here are some further considerations:


A good thesis topic should not be all or nothing.  Instead, it should accommodate a range of levels of ambition.  There should be fallback options that would be enough to graduate even if things go poorly, moderately ambitious outcomes that you could reasonably aim for, and very ambitious outcomes that would be fantastic if they could be achieved.
It should offer scope for creativity and choice by the student.  You can only impress hiring committees so much if it looks like your advisor was the mastermind behind your thesis.  Instead, it has to be clear that over time you took ownership of the project and played a major role in determining its direction (even if your advisor made the initial suggestion and provided feedback and guidance).
Your thesis topic should connect with other topics you'd like to explore in the future.  If you spend your entire early career revisiting and elaborating on your thesis topic, it won't set you up for success as a researcher.  Instead, you need to branch out, and your choice of thesis topic can really help with this.  Choose a topic that is central and well-connected to adjacent areas, rather than one that is isolated.  That way you can build on your previous experience as you branch out.
Your thesis advisor needs to care about the topic.  Sometimes people are willing to supervise theses on topics they don't particularly care about.  They do this in an attempt to be helpful, but it can end up working against you when you graduate and discover that your advisor's lack of caring translates into a lack of support on the job market.
For a multi-year project like a thesis, it's generally best to pick a topic that seems to be becoming more popular and influential over time, rather than less so.  This sounds obvious, but it's surprisingly subtle to figure out.  Beginners are sometimes a little behind the times and can get excited by topics that were hot a few years ago but have since been thoroughly investigated and are already in decline.
It should be something you like doing, not just something you would like to have done.