Pictures. Let's face it, porn revolves around pictures. That's why it's so important for a paysite to have good quality pictures, or it'll fail. The photos should be crisp and clear, not blurry. Sure, there may be a couple photos here and there that may not be up to snuff, but at least 98% of the photos in every gallery should be crystal clear.
In order to thoroughly enjoy these photos, I also think porn sites should have a slide show option. Porn surfing is often a one handed activity, but the slide show option would further enhance most paysite members' enjoyment. Who wants to have to stop and click to the next picture with lubricant-sticky fingers? Ideally, a paysite should have a slide show option where a porn fan can choose the timing of the slides. I understand some porn sites are new and may not be able to afford a web programmer to set up a slide show. If that's the case, then I hope they make it easier for the paysite members by having a "previous" and "next" link to the rest of the pictures in the gallery. I've seen some porn sites without that option; this is definitely not user friendly for a porn fan.
The zip download option is another key feature for a pictures. Many members want to enjoy the photos well after their membership has expired. The ease of downloading all photos at once in a zip file cannot be stressed enough. If paysite members love a particular photo set and/or model, they don't want to have to spend time saving most, if not all, the photos in a gallery. They will remember the user friendly paysites with the zip download option and will more than likely continue their membership or rejoin in the future.
A lot of paysites offer various pixel sizes for each photo. I like that option, although the paysite needs to ensure that all sizes have crystal clear quality. On the other hand, I'm not sure how user friendly 2000 or 3000 pixel sized pictures are. I don't particularly care to see my whole computer screen filled up with one eyebrow - lol! But I suppose if a porn user has a fetish for gyno, asshole, or nipple shots, these bigger pixel sizes would be quite welcome. Whopper pixel sizing fills up their whole computer screen with their fetish!
Speaking of size, I think thumbnails should be bigger on a lot of paysites. Many of them are so tiny you can't see what the model looks like. If a paysite only has room for tiny thumbnails, then they should just show a face shot. Many porn users like a pretty face almost as much as they do a hot body. There's no sense in putting a full - or mostly full - body shot on a thumbnail the size of a postage stamp or smaller. How can you see what the model looks like? I don't know why these porn site don't just make more pages of thumbnails, instead of trying to squeeze a bunch of these eensy weensy pictures on a gallery page. What's the reasoning behind very small thumbnails? Is it to make a gallery page look like it's full of a whole bunch of content? Well, 50 thumbnails are 50 thumbnails, no matter how many pages you span them across. I think it mostly frustrates porn fans, because we waste our time clicking on models we may have no interest in.