For the gay romance and erotica, it's whether the men act like men. And more specifically, whether they act like my somewhat-informed picture of what gay men would plausibly act like -- I know there's a full range of possibilities there. I have people I can consult on these things, too. ;) The sex scenes are more often than not what I would picture as relatively correct. The character development and the characters' actions out of bed are more what I hold to a particular standard. I think relationships between men often have a certain level of competitiveness, too. Men don't have the same issues women do with competing, arguing, and playing dominance games. There's also that visual appeal moreso than the emotional appeal -- at least right away. If a story with gay men has them slowly courting each other over a period of months, and extensively analyzing the emotional aspects of the relationship, I'm likely to think that odds are high that that story was written by a woman.
For the bi stories, I have a more personal viewpoint. I feel pretty free here to just enjoy the stories from the perspective of what I think is romantic or what appeals to me. Although I can't speak for bi men, I've known my share of them, and again have that mental picture of how they might act. Much less romantically than in most of the stories, in my opinion.
For the lesbian stories, I also judge by what appeals to me. The meaningful conversations, not so much, although I'm sure that's a huge factor for many women. Characters agreeing so much that it seems like they blend their personalities and are almost instantly one emotionally -- yes, realistic enough, actually. Doesn't work for me, but, again, works wonderfully for so many women. These are the emotional developments that I find so implausibly in male/male stories -- I just don't think that most men have that emotional blending, especially so very quickly, in the way that a large percentage of women do. The way most women work so hard to cooperate rather than compete -- that's never come naturally to me, but it certainly does happen for many. Sex scenes: basically most seem to be pretty correct as to the typical range of activities. Although I always remember the line from a college friend: "It's not really lesbian porn unless the women have short fingernails." Just another little thing I consider in my mental checklist for realism. Some lesbian romance and erotica really does appeal intensely to me, but it has to be more what I enjoy on an emotional level, too.
For heterosexual romance, I've read so much that the story has to really stand out for me. I want the men to act like men, and the women to act strong and independent themselves, not like little whiny dishrags. Again, I judge by what appeals to me. The trend in heterosexual erotica stories for the characters to have anal sex -- is it just the current "pushing the envelope" thing to do? Is it that trendy in real life? Bondage is another one. I don't think I've seen one of those stories with discussion of a safe word, or a character checking to see if a bound character has enough circulation left to important body parts -- his or her hands, for instance. I know it's romance, but attention to the details is important, too.
Menage stories -- a lot of these have situations I would think would be painful in real life, but forging ahead regardless...As I've mentioned, if it's m/m/f, I enjoy those rare stories where the characters seem to be genuinely bisexual, and all enjoying each other fully. Straight men both enjoying a straight woman, but not each other -- it doesn't appeal to me as much, although apparently that fantasy works well for a lot of women. The men don't touch each other, and that seems sad to me. The socialization in contemporary America that doesn't allow for straight-identified men (really, men generally) to be physically affectionate with each other except under strictly defined circumstances seems so limiting, especially in stories like that. Going the other way, I've posted on this particular plot before, but to have an established gay couple suddenly including that special woman doesn't really ring true to me -- although I can see why that is also a cherished fantasy. Do the menage stories have wide appeal? Apparently they appeal to a large enough percentage of readers. I think that in real life, it's difficult enough dating one person at a time, but for these stories, I'm like, hey, it's fiction, rock on.
Really, what I want is for a story to have good enough characters, and be written well enough, that it just sweeps me up and makes me happy to go along for the ride. Certain aspects of the story need to be realistic enough for me to suspend my disbelief generally. And the stories need to hit certain chords for me. For the straight romances, I know from the blurb which stories will do that for me if they're well written, and have appealing characters. For the other categories, I'm learning which types of plots really work for me. It's still harder for me to tell from the blurb, as there's more variety possible. But I have pretty clear ideas that way.