Kate's banter is probably my favorite part of the show. It is an excellent counterpoint to Clint's much darker and brooding storyline and character. Together, they made a excellent team (even with its off-again, on again moments). However, it's her developing interactions with Yelena in the last couple of episodes that was my absolute favorite part.
My biggest complaint is that everything gets wrapped up too quickly at the end, leaving me with a number of questions. I was particularly disappointed with Echo's arc. In a way, it reminded me of the introduction of the Punisher in Daredevil season two, but with even less development and closure. It's clear that Disney is setting her up for her own show, but she felt shoe-horned into this one.
All told, this was a great Christmas-themed show that I could watch again and again.