Plot holes mean a problem with the plot. Claire going or not going tot he hospital or no one seeing Claire's head while she was on the ground are not "plot" holes, at worst they are highly unlikely events that happen to have happened...
It is very likely that one of the maybe three people who saw Claire get hit would have seen her head spin, it's also possible that they were all busy with practice and that no one noticed. While that may seem unlikely, while you may have a problem with it, to say that it's a plot problem suggests you don't know what a plot is.
Also, Heroes has one of the highest production values and intricate plots on TV right now. It's not perfect but it delivers powerfully at least three times an episode. It's one of the best, and while there may be some show that I haven't seen that's greater ( Lost, The Office), I don't see how length of run could make a lower quality show somehow "Greater"