Well Kizaru's ability isn't really a plot hole since he is a light man not a speed man like Quicksilver. For Kizaru to move at the speed of light he has to become light and as we know light has no rest mass, so as light all he would be is a giant laser beam. That is essentially his ability where he gets somewhere instantly but in order to attack he has to revert back to a human body. That's why his kicks etc are not pure and utter destruction of the scale that would occur had he actually moved something with mass to the speed of light essentially breaking the universe, since he only turns part of his leg into light to accelerate his leg(not to the speed of light itself), making his kicks really strong. The only way he could truly kick at light speed is if we drop real world physics and go by OP world rules, in which case you can no longer argue about it being problematic as Oda can make up whatever physics he wants. In addition, since Oda has not provided us with a formal set of rules for how OP physics works we can't say something is a plot hole when he does something unless he previously established a specific rule and broke it later without any explanation.
The things that I see as more of a possible plot hole is what @MBVC mentioned about how Kizaru hit a key with a beam when Luffy's head is a bigger target and would have solved the problem and killed Luffy, which the marines wanted to do. Now someone can argue that Kizaru is not a killer, but that would just seem very strange for an Admiral, who has been doing this for decades and probably has killed etc. I mean the marines even release bounties saying dead or alive, so it doesn't seem as plausible to say an Admiral would rather not kill when his orders were to kill at the beginning of the Marineford War when Kizaru specifically asked for permission to execute
http://www.mangapanda.com/one-piece/557/14 and was then given permission. To me this counts as a plot hole, but a reasonable one that allows plot to continue because in a fictional world, things like this just have to happen to make the story exist and continue.
