1. People - always the most important factor to consider on projects. Do you like the people, are they people you admire, do they have knowledge that you want to gain etc.
2. Industry - Some industries I was not always interested in, others I wanted to learn a lot more about!
3. Type of project - I prefered strategy projects but others liked execution/PMO type projects more. Some were analytical, others less so etc. Everyone has their own set of preferences
4. Location - long distance travel sounds fun until you realize it can get taxing very quick
5. Duration - short is not always good but neither is too long.
Those were some of the factors I used in my decision matrix