It's difficult to answer a question like this, because student's life depend of the city you study in. I'm french but I study 6 month in Colorado (or kinda), and in my opinion those are the main differences between studying in the US and in my country :
1-French Universities look poorer than american ones. When I was in the US the campus looked very pretty, with well-maintened roads and buildings...In France universities lack money, and so look poorer (on the other hand they are free). Of course it depends on the city and the field of study, some particular prestigious departments (engineering, commerce) may get a lot of money.
2-French tend to work a lot less than american in University. As there is no fee, students tend to be a lot less motivated to study (few have a student loan), at least in the first years. Not attending the class is common. It changes in the last years, wether the students become more serious or the non serious ones have been kicked off.
3-Administration and bureaucracy. It's France, so be prepared to fill form of any sort and to deal with the migthy administration...(I was astonished when in the US it took me only 15 minutes to get my social security number!)
Well, that's what I think of...for the rest (like going out, visiting, partying) it depends a lot on the city. Of course there is a ton of cultural differences like eating (you get a break of one hour to eat), relationship with the teachers, housing,...
If you have more specific question you can email me : seb_421 at yahoo.fr
Oh, yes, one question : why for god's sake do you want to go in Lille? Are you fond of gray sky ;)? Frankly it is not the best city to live in in France (...it may even be the worse as far as I know).