Kalista isn't broken or OP
It's just that she fits the mobility meta so well that it makes her seem far more OP than she really is. I fear that due to the amount of QQ from the community, Kalista will be "Olafed", further killing diversity in League. While I do agree she needs nerfs, especially her pseudo-smite, they really should only be love-tap nerfs to bring her in line with the rest of the ADCs who honestly aren't too far behind.
All of the complaints I hear about her is that she's too mobile and uncatchable, but people need to realize this is a problem with the meta in that mobility champs are far superior. Put Kalista up against a super-mobile team, and she's relatively easy to deal with, but against a team with some non-mobile champs, and she is definitely problematic. However, this is an issue with mobility creep and how Riot has no idea how to create balance between mobile and non-mobile champs.
I still think Riot needs to put a cap on her Rend stacks as well as maybe a tiny distance nerf to her dash, but I still believe she is only powerful and complained about due to the mobility meta, which at this point it seems Riot wants to keep this meta.