While loving the science is important, it is not the only factor that goes into picking a lab. You need to be happy with the lab environment or you will end up hating the science you initially loved so much.
Your mentor is one of the most important components to the decision. You want a mentor who will support you no matter what. Every mentor has a different style. Some will let you wander and experiment for weeks until you ask to see them. Others will stop by your lab bench several times a day to micromanage you. You need to find the style that suits you best. Each has its own advantages and disadvantages. The best way to do this is on rotations. If you are lurking around the lab trying to avoid your mentor, this is not the place for you. You should also use the rotation to ask lab members about what happened to former graduate students and postdocs in the lab. Did the students graduate in a timely manner? What sort of publications did they have? Did the postdocs head on to good jobs? Were there any fights or incidents? These are things you need to know. Make sure you are comfortable talking to this person and that they will set you up with good projects and help you graduate.
Lab environment is also a key component. While lab members come and go you want to make sure there is no person in the lab that irks you so much you will hate going to work. Are lab members overly competitive with each other or do they support each other?
Another factor i slab set-up. Some labs have lab managers and technicians who organize things and take care of a lot of busy work. Other labs do not. While this is not a huge deal, have an organized work environment helps you get your work done. When people are fighting over supplies and things are in disarray it is tough to work. If this is an animal lab - who does the gennnotyping? Who takes care of the animal cages and deals with the animal facility staff?
How flexible will your project be? Will you be allowed to follow your own interests or will you be forced to do what the PI/lab wants done most? Are there financial or spacial constraints in the lab that will prevent you from following your own ideas?
I had two excellent PI's, one young and starting, one more established and experienced. Both lab environments were friendly and organized. Since the MD-PhD is long process I chose the more established PI. He can help me graduate faster, his technicians are wonderful and the science in his lab is more clinical and thus I can relate to the research more. It was a very difficult decision and not one I took lightly. I spoke to program administration for advice as well as more senior students and friends. I weighed all this advice and the pros and cons of each, and finally I went with my gut. Let's hope the next four years proves it's the right decision.