It's somewhat hard to define, but an "alt" girl deviates from the normal porn look. An "alt" girl has a tendency to have dark hair and tattoos, piercings, and in general, an unconventional look.
It's all very subjective in the end though.
I equate alt porn as the punk rock of porn.... rebellious and unconventional. But it means different things to different people of course. Like, for some it can be seen as the avant-guarde movement in porn.
Joking aside, and let me indicate that with a big f'n sarcastic smiley face , let's just see people making the kind of movies that they would like to watch themselves. Enough of the focus group. Enough of labels. Enough of movements. You should know that in any scene it's - usually - the originators that are worth bothering about, not those riding the coat-tails.
Good movies of any description come from a personal vision and if enough people share that vision then you've got a hit. If you really want something genuinely "alternative" to most porn, then let's stop people just jumping on the treadmill and start demanding that they get involved in producing movies that actually means something to them.
Personally I think there is a big difference between Joanna Angel and Eon McKai (or Jack the Zipper). Joanna has a light-hearted take on sex, trying to make sex look like fun, whereas Eon's take on sex is much more angst-ridden, often it seems that he is trying to make people aroused and slightly troubled by their arousal at the same time. Unfortunately it is McKai that is usually seen as the example to follow, but what would be intersting to see is more people exploring the kind of stuff that Joanna is doing: porn that looks different without being depressing.
There is delicate balance to be struck between porn looking cool and porn being arousing. Although I usually like alt-porn stuff, sometimes I get the feeling that the director's chief concern is with being cool. So the obvious answer is that while the alt-porn style is hot it has to be used with the objective of making the sex hotter, not just as a way of showing off with respect to your myspace buddies.
This touches on the biggest problem with alt-porn, that the sex featured in it is usually the same-old routine that we get in other flicks, just with different clothes, make-up, sets, and lighting. We get the atms, the dps, the gagging, and so on, the same stuff usually served the hard-core gonzo crowd (although they certainly prefer it to be presented in a more straightforward way). Many people that are once-in-a-while consumers of porn find these things gross and I think only a few of them find alt-porn to really be that much of an alternative. So I think there is a potential market for what might be called alt-porn light, namely all those people who don't think sex has to be extreme in order to be hot, but who also dislike the look of standard porn (i.e., women as either barbie dolls or petit teens, men as looking like they're on steroids and far too often old enough to be dads of the girls they fuck). And this is a marekt that current alt-porn providers do not cater for at all. Traditional porn companies try to cater for it with "couples porn", but I really doubt that they capture the entire relevant demographic with that.