Just like the fans it's the same boohoo they did this with this character and so forth. I would like to hear some actual criticism, if you have no sentimentality for these characters it means nothing. Hamill's just probably disappointed that he can't hog all the spotlight and that his character isn't infallible.
Actual criticism? Luke Skywalker´s characterisation was poorly written. The film shoved him into a role it wanted him in without much care for his core character traits and little to no explanation for that change. I don´t even have that much of a gripe with his personality on that island (although his insistensy to be left alone to die after leaving a freaking map behind to find him had me scratching my head). It is how he appearantly got to his state as hermit that felt incredibly clunky and thus was really hard to believe.
If I would have to summarize Skywalker´s character in one word it would be "hope". Hope for a better future but also the belief that pretty much everyone can be saved, even from themself. This belief led him to basically go on a suicide mission in EP VI in an attempt to redeem his father, a man who has done unspeakable things. Skywalker was shown to be a very reluctant killer even when presented with grave danger. This point was made in the clearest way in his lsat fight vs. Vader and the lengths the latter had to go to make Luke unleash true killing intent. It took Vader to enter Luke´s mind and then threathen to turn his beloved twin sister towards the dark side.
Now enter the "birth" of Kylo Ren and the best explanation for Luke having the urge to kill a defenseless child (his own nephew on top of that) in his sleep the writers came up with was instinct, because he saw Ben´s potential for the dark side. That is an incredible stretch and simply something I can ever picture Luke Skywalker doing, not without prior development towards that point. It was a cheap plot device to force Luke outside the picture.
For the record, this whole point did not ruin the film for me (it did lower my personal rating for the film, though) as it had several strong points (beautiful shot, great soundtrack, two very well made and entertaining large scale battles) and if you don´t dive into the plot deep enough, it was a good blockbuster. However, I´m also not what you would call a real fan of the series. I´m too young to have been grown up with the original films, having seen them only three times in my life. If I put myself in the shoes of the people who love the first three episodes and it´s characters, having grown attached to them through dozens viewings, I can easily picture the ire they developed towards that film. I can even understand why they would rate it below the prequels as in the difference to them this film actually utilized their beloved characters (and advertised greatly with them) only to turn them into characters they simply don´t recognize anymore.
The dominant reactions of the Star Wars fans I know was simple: pretend this film never happened and ignore Ep IX.
