I honestly still think she's a bit unhealthy, but I wouldn't argue she's OP or needs immediate attention. Her kiting ability, scouting ability, and rend's interaction with epic monsters are my main problems, and I'll go into detail below about why those, in particular, bother me.
I realized, while talking about Yasuo yesterday, that one of my problems with champions in this vein is that they put a TON of counterplay burden on the opposite team. I just fail to enjoy certain champions that are so unique that their mere presence warps the game around them. It's absolutely a matter of opinion, but Kalista is one of those champs for me.
She utterly warps the way the game around epic monsters works, mainly. You have to constantly be aware of them in case Kalista is there (jungler or no). That wouldn't be a problem for many adcs, but Kalista has EXACTLY the kit to make that a horrible experience. First, she has the scouting tools to help her stay especially safe while doing that compared to other adcs; second, she has unparalelled kiting ability, so that the usual counter of "go ham while they are doing the baron" (or drake, or what-have-you) doesn't work as well as for other adcs.
And yeah, I know that "take cc" is the counter to that. But even still, this isn't happening in a vacuum. The fact remains that she has an uncommonly strange set of strengths for an adc. When Kalista is strong, she warps the meta around her more than other adcs can. I'm not saying she makes the game less fun... but she's the least fun adc, imo, solely because of how unique she is.
Again, just my two cents.