Probably the amount of 'down-time' there is. From the outside it looks as though people are constantly studying or socialising etc., but actually a good chunk of your week will be spent in your own company - either doing laundry, cooking, cleaning, food shopping, killing time between lectures etc. Sometimes none of your friends will have free time when you do, so don't be afraid to do things by yourself.
As others have said too, don't expect to instantly make friends or for people to seek you out - be proactive about it, and you should be fine.