I'm only an intern right now, so my advice is somewhat limited. However,
I was just in your shoes just a year ago and right now I'm doing ortho, and all the rotators come through my service and work somewhat closely with me. From what I've heard from my seniors, I can say that at my program, residents do have a big say in the rank list. We may not be able to say who gets in, but we can move students up the rank list somewhat, and we can definitely move people who were at the top down the list, or even off the list.
What I'm looking for, and I think this is the same as the other residents, since I"ve discussed the students with them, is what kind of person are they. Is the student someone I can see myself working with, or being my junior resident. Do I enjoy working with them? Do they do things to make my life easier? Are they enthusiastic? Are they a team player? Are they still somewhat humble and respectful? Orthopaedic knowledge isn't huge on my list because that's what you're here to learn, but it's nice to know or see that they've read some things and know a little bit. In the end, just work your ass off because that's all you can do. Not every one fits in everywhere, but you can definitely still get a good letter at all the places you rotate if you work hard.