It is going to vary a lot program to program, but you will not be relied on as an independent thinker from the start, as you might be as a PGY-2 IM resident (at least on some rotations). That is both good and bad, because for a lot of things, you are now fully capable of working things out on your own, and it can get frustrating to be put back on a leash. On the flip side, you aren't going to know much detailed neurology when you show up, so it is good to be able to take a more passive stance for at least part of another year. You'll gradually start taking more responsibility, and before you know it you'll be running your plan by your senior rather than calling for orders/advice. It's a really cool progression.
I totally second the sentiment that while just as hard or harder than PGY-1 year, it goes down a lot more easily because a) it is what you want to do with your life, b) you can see a direct application to each teaching point and case, and c) fewer FTT admits.