First impressions are a major factor in whether an AIF game is a success with the audience. When a player first starts up a game they are extremely easy to put off. It will be rare that a player sits down with any dedication at all to really stick with a free porn game and see it through to the end. They'll almost always be thinking, "I'll give this a try for a minute and see if it interests me." Therefore you need to get them interested in that first minute or two and not push them away. This might be the most under-appreciated element of good AIF game design.
Show some leg
You don't need to do a full sex scene right away, but you want to signal to the audience that this is a sexy game. That's what they're there for. Even if you feel the main appeal of your game will ultimately be the story and characters, you need to accept that initially your audience primarily wants some sex. You can draw them in to your writing gradually as you go, and you don't have to make an out-and-out sex romp, you just need to get the player's engine running.
This early 'showing some leg' doesn't actually have to take any particularly specific form. Maybe you've got a shortish game and all you do is show an attractive character and maybe hint at what kind of sex might be coming up (like if you feature a dominatrix in some capacity at the beginning, you are probably going to have an S&M scene later). Judge based on how you want to pace the sexual content throughout your game - how much do you think you would want early in the game if you still haven't made up your mind to keep playing?
It's probably also best not to have the early sexual content be completely satisfying. If the player is... erm... "satisfied" then they will probably stop playing, at least for a while. Oh, and it's better to have no sex at the beginning rather than boring sex - if you don't have anything good to include then don't include it, because like I say, this is your first impression.
Establish the look of your characters
This is strongly connected to the above principal of 'show some leg' and is probably generally good writing practice anyway, but it is particularly important for AIF.
A surprising number of authors fail to strongly establish the physical descriptions of their characters early in the game. This is always one of the first things I look for - do the characters interest me physically? Not to put to fine a point on it, is this girl hot? This is offset somewhat if you are using graphics, because I might be able to just look and see for myself, but in that case you must a) make sure the pictures do a good job of establishing the look of the character, and b) still make sure you describe them clearly anyway.
When painting a picture for the reader throughout the game you want them to automatically have a sense of what your characters look like. Personality is a more subtle thing and can be built over time. It's worth mentioning here that you should consistently reinforce the description of the character throughout the game as well, don't just describe them once and forget about it. There's more about this in the discussion below in the comments section.
Don't overload your intro with exposition, back-story or just too much text
Try to get into the flow of the game as quickly as possible. You have time to flesh out your setting and characters - the player doesn't assume they know everything when they first start typing commands. You'll find opportunities to introduce stuff - exposition and back-story will most likely come out organically as you write. You don't need to tell the player right away that the PC was a star high school baseball pitcher if they're going to be throwing a baseball a short way into the game. You can just tell them when it comes up.
It can even be a good way to give your audience a nice early surprise; let them think your game is doing one thing, then after a little while reveal what's really happening - but only when you have to. Say the game starts in the middle of a dinner party, everyone is being polite and you have an attractive date that you would like to impress. The player doesn't need to know right away that you're also having an affair with a married person at the same party - they can find that out when the two characters briefly get some time alone and the married person suddenly jumps them.
You don't want to leave the exposition until really late, either. Raiders of the Lost Ark is the only case I can think of where a major, out of the blue plot change is revealed at the end and it still works (What? The Ark really does have magic powers?) If you can, try to move early exposition into small early scenes instead.
Give your player some freedom or control early
Further to the last point: the player doesn't want to be reading a lot of text the moment they get into the game. They want to be in control - they want to be playing the game as soon as possible. Not reading the game. This means a bit of interactivity. Some of this interactivity can, (and probably should) be introductions to game mechanics (explored more below), but you also should give your players some meaningful interaction. Some kind of puzzle or some kind of choice early on is a very good thing.
Maybe the choice is "do you want to be respectful of this nice girl, or be smooth to the easy girl?" Or maybe it's "do you want a bit of money or a chance to see a naughty picture?" Or perhaps it's an interesting little puzzle where you can get a cool bonus if you complete it quickly. Just give your player some agency and the chance to do something other than just 'push the story button' to watch your story play out. If this means introducing some small extra bonus content on top of your story to have some kind of reward to dole out, then you might have to do that.
Teach the player the basics of how your game plays
It's important that your players understand roughly what is expected of them. You don't have to start at the absolute basics for most AIF games, although if you think you might be attracting first time AIF players then it's actually not a terrible idea to let them know about movement commands and 'x' for examine, etc. What's important for most AIF games are certain things like what conversation system you're using ("ask", "greet", "talk to"?) and how important it is. Are you using any unusual game mechanics? That kind of thing.
And try to demonstrate their use by actually having the player use it in some simple situation. I'm sure you've seen this kind of thing done in game design before - big professional games do it all the time these days. "Here's a log in your path - press space bar to jump over it." It's in some ways a tedious trope but AIF is a niche form of game design that many players will not know very well, and thus you need to do a little bit of hand-holding.
Broader acceptance of this kind of stuff might actually help the community as a whole by making the genre a bit more widely accessible.
Give your players a goal
It might be obvious enough to you what the player should do next, but it may not be obvious to them. Sticking players in a room and assuming they'll go out and explore may have worked fairly well in many good games before, but that does not make it good practice. If you want your game to be well received give your player a reasonably clear goal at the start. It doesn't have to be the player's end goal. In fact if it is then you've probably just had to unload a wall of text on them to explain the entire plot at the start. Instead, give them a simple goal explained by the circumstances.
I much prefer it if not very much is expected of me at this point. That is, I don't have to efficiently discover some item hidden in my bedroom within the first four turns of the game or miss out on later content. What makes a good opener is an early setup that fulfills several of the tasks described here at once: say in a superhero game, if you start out in a very easy battle you could give the player a goal (defeat your enemy) introduce some basic gameplay mechanics, give your player some limited control (maybe they could choose whether to save a bystander or arrest the bad guy at the end of the fight?) and explain quite a bit of the story concept just by implication, without having to write a long passage about the fact that you have superpowers, blah, blah.
Have unanswered questions
This is a well established writing principle. You want the early part of your story to be simple and leave the audience with the impression that they'd like to work out what's going on. This isn't rocket science: a man walks into a room, where his workmate is working away at his desks. He asks a few boring questions about how his work is going, how his wife is, etc. Then he pulls out a gun and shoots his workmate dead. Why did he do that? Read on to find out...
It's best if you don't answer those questions right away, either. This gives a story a "page turner" quality. It really doesn't have to be that intense, either. If you've played Pervert Action: Future (sorry to cite my own games, it's all I can think of right now) then you'll know I start off with a bit of a prologue about Kenji being recruited for a mysterious "unit program", and then the opening scene has he and his friend Ami setting off on a shuttle ride to the space station. I was trying to set up a bunch of little hooks here: why has Kenji been recruited and why is he so special? What is this "unit program"? What is this space station going to be like? Can Kenji get anywhere with this cold but sexy military woman? And so on.
In some respects this principle is actually slightly less important in writing AIF than in writing, say, a novel. The reason is that as I stated above, the player is initially mostly there for the sex. So long as the game looks like it's going to have some hot action the player may well be fine getting through a relatively slow start to a story (Crossworlds, I'm looking at you). But it nonetheless still helps, and it's worth respecting this basic element of writing craft.
I think there are quite a number of AIF games out there that have not been accorded their due because they don't grab players from the start. I've also been frustrated before by games that I wanted to enjoy, but which made me struggle through them at the beginning. While good games in AIF have consistently bent or broken these guidelines I don't think that means they are unimportant - just that we don't have much of an 'industry standard' and players interested in AIF will endure a slow opening for the chance to enjoy an AIF game.
To emphasise the point, here's another article by ExLibris on the topic of the importance of how your game opens. He explores some other stuff like spelling errors in the opening (that is important), the title and what the player will do when first starting a game, so it's worth a read.
Is a first impression as important as I think it is? Did I leave anything out? Can you suggest good examples of games that demonstrate the principles listed above?