State of LeBlanc - A Need for Change
Introduction Facts will be used in this post, not opinions or anecdotal experiences with the champion. A TL;DR and Sources section will be at the bottom. I am not arguing she is overpowered or underpowered but rather that her DESIGN is abysmal and needs actual changes. I intend only to facilitate the need for change. Without further ado, I have three primary arguments. The first is the scarcity of counterplay, the second is unequal effort and the third is a violation of standard power curves.
Insufficient Counterplay The average human reaction time is approximately 0.2 second. After this, you must decide which skill/item to use, aim/target LeBlanc and activate it before she teleports back out. Keep in mind her combo can take place in .4 second. This is all assuming that you ANTICIPATE LeBlanc coming. If you do not, your reaction times will suffer considerably. This means that, potentially, it is not humanly possible to react in some instances. Lastly, while she may have lane counters, Riot has taken the stance that counterpicks alone aren't sufficient, as bans exist, as do counter-counter picks and lane swaps. On top of this, what if your team isn't mid lane? She makes your entire game miserable even if you didn't lane against her. This is an oppressive feeling that Riot would likely deem toxic.
Unequal Effort This leads to my second argument. LeBlanc's effort compared to the effort that has to be exerted to stop her is completely unequal. Your reactions have to be nearly perfect on top of seeing her coming to reasonably react to her combos. LeBlanc on the other hand has ridiculously low cooldowns, several panic buttons and CC. She has obscene burst and can completely negate your chance to react to her. In fact, her kit revolves around it (As is reflected in the fact that she used to have a silence as well). All LeBlanc has to do is her same combo every single time, which isn't particularly hard to consistently pull off.
Power Curve and Design Finally, her power curve. Everyone knows she has disgustingly strong power early game, she is infamous for this. Like most mages, she is also extremely potent mid game. Frustratingly enough, she scales well with her snowballing lead should she acquire one, which is quite unlike most early game stomp champions. This is another reason she is so frustrating to play against. Even if you stack magic resistance (Which has poor itemization), MPENN has fantastic itemization. If she doesn't 100:0 you, she can simply teleport out, wait her ~40 second cooldowns out and all-in again. This is, again, not a result of pure numbers but champion design. She can just ignore your input and just do what LeBlanc does, successful or no, and get out Scott free too frequently. Darius was nerfed for having his entire kit back up to re-bully you out of lane every time he killed you. LeBlanc has better scaling than Darius, better mobility, less counterplay but gets a free pass? That is highly inconsistent in philosophy.
Conclusion All in all, whether she's overpowered or not, it's quite irrelevant. Her design is simply horrendous. Poor counterplay, poor power curve, poor kit design, horrible design philosophies and band-aid fixes were all rolled into one to produce such an abomination of champion creation, and it most certainly meets the standards for needing changes. LeBlanc's current state is highly unsatisfactory.
TL;DR/Summary LeBlanc's kit innately denies counterplay and defies traditional templates for an early game champion and needs changes regardless of current power level due to an oppressive nature that Riot has already taken a stance against.
Sources http://biae.clemson.edu/bpc/bp/lab/110/reaction.htm http://www.humanbenchmark.com/tests/reactiontime