The tricky thing about erotica is that, although I've heard some agents and publishers refer to it as a subgenre of romance, it's still a wide umbrella term for everything from Pauline Reage's
Story of O and Anais Nin's
Delta of Venus to Tiffany Reisz's
Original Sinners Series to very short stroke pieces that may or may not contain reptiles or bigfootlike humanoids. Sometimes those readerships overlap. In the case of the reviewer you mentioned, perhaps not. I don't read a lot of short fiction in general, but when I do, I still like there to be some plot and characterization. I'm far more likely to pick up a copy of
Best Women's Erotica than I am to take a chance on a 99cent short. I also write erotica and erotic romance novels which contain a great deal of plot and character development, probably because that's what I grew up reading (see the aforementioned Reage and Nin titles, plus Anne Rice's
Sleeping Beauty Trilogy).
As for what erotica is and isn't, far be it from me to try and define an entire genre. My understanding is that with regard to erotica vs. romance categorization, erotica focuses on one character's sexual journey, while romance focuses on the couple (or thruple, or whatever). That said, one of the agents I follow on Twitter was recently lamenting the lack of Heroes in M/F erotica queries. So...
I think it's possible for erotica to have too much sex, though I don't mean that in general terms. If a book sets itself up so that it appears as though it's going to have a lot of plot and then doesn't, it's going to feel imbalanced. Likewise, I just finished a book that started off as erotica and then attempted to become erotic romance, but since there wasn't enough time spent on what ended up becoming the central relationship, I didn't buy the happy ending. I think it's about doing what's right for the story (isn't it always?). If it's a short stroke piece, too much plot or characterization can bog down the action. In that case, "Show, don't tell" becomes much more important, and fortunately, sexual preferences and behavior can reveal a lot about a character.
Wait... If a story has a lot of plot (that doesn't involve the romance or intimacy of the main relationship) than sex scenes, isn't that an erotic (include genre here)? Like erotic romances, etc?
One would think so, right? But I almost never hear of anything erotic classified as something other than erotica. If people are banging a lot and there isn't a central relationship that ends in a Happy For Now or Happy Ever After, it's erotica. Even if people are getting shot. Even if they're just filing their taxes. My first novel was what I considered to be speculative erotic suspense, but if I were to ever let it see the light of day again, I'd probably just query it as erotica.