Hmm...I guess that you know a lot more of the people? Pretty much everyone knows everyone in the grade, and a lot of the people in the grades above and below. Although depending on your personality, that could also be a downside

But at least you don't have that whole "we went to school together for four years and I have no idea who you are" thing going on. And it's probably a lot calmer without so many people running around between classes!
There's generally fewer course options though, and scheduling is trickier because each one is only offered once or twice which is the major downside.