The reason normal femdom doesn't get me going as much is that there's usually no real domination. In most femdom hentai (and virtually all femdom 3DPD) the guy is willingly letting himself get dominated because he's a masochist. Sure he's being physically hurt but he's using the girl to his advantage, arguably making him above her in a way, since even by putting himself in a compromised position he's still getting what he wants.
A lot of "normal" femdom doujin and even some size stuff include the cliche>I'm bullying you but you're getting hard? What a pervert~
line, and I don't know about anyone else but motherfuck that is such a turnoff and I don't know why it's so common. If the guy's enjoying it that throws the whole purpose out the window for me. Yeah in some cases he's obviously getting aroused despite not wanting to and that's humiliating in its own right and whatever, but it's still much better to me if the only one getting aroused is the sadist girl.