Many have criticized pornography as exploitative; there is plenty of anecdotal evidence that some workers are not treated with respect or dignity, particularly by certain producers.
More recently, though, there has been a move towards increased labour rights and respect of performers. The top performers have always made a good living and are typically treated well. The ethical pornography movement takes it one step further. A core part of the approach is that performer rights are paramount.
There is also concern that pornography negatively affects consumers. Porn likely affects different people differently, and type of content interacts with intrapersonal characteristics. The research shows that the consumers who we should be most concerned about are those (men) who are characterized by hostile masculinity (e.g., attitudes supporting violence against women, rape myth acceptance, hostility towards women, etc.) and antisocial traits (e.g., impulsiveness, low empathy, etc.). They're the same viewers who are likely to choose content that represents the worst of the porn industry (e.g., exploitative, misogynistic, etc.). For these men, porn use can increase risk for sexual violence, and also feeds their nasty attitudes about women.
Another concern is that porn is replacing sex ed for teens. Sex ed in some geographical regions is great; in most others, it's still pretty terrible or superficial. If teens watch porn without sex ed to balance it out, their blueprints for sex and relationships will end up being based on the fantasy world of pornography, and not reality.
As far as we can tell, though, the vast majority of porn consumers reject the type of content that is most problematic, and now that porn has become ubiquitous, many have argued that the industry is moving in a more positive direction (not all agree, however). There's also lots of new research coming out showing that porn consumption is related to all sorts of positive outcomes and experiences, despite the narratives you hear in the media and online.
I've already discussed ethical and non-ethical porn on the blog previously so I'm going to pass it over to Violet Blue for her insight. If you have the stomach for it, contrast her recommendations with the content produced by people like Max Hardcore and Khan Tusion.
(VB's recommendations at the bottom)
This is Violet Blue: