Subscriptions are a big one. But I think in a surprising sense the industry is moving away from sites reliant on subscriptions (e.g. people forgetting about it) and you are starting to see stuff that is actually worth ponying up for (at least in comparison to older model sites).
The biggest factor though, from my limited experience would be the impulse purchase factor. If buying a soda in line to check out at the big box store is impulse, then porn is hyper-impulse. There is generally little to no planning going into "I'm going to go spend some time on a porn site right now", so you need to have as frictionless of a transaction as possible.
Bitcoin just doesn't reach the ease of use of credit cards yet. I think it can, but right now if $random_40_year_old wanted some bitcoins NOW so he could check out some adult site he found, how does he get them easily and quickly? The answer is you don't.
Once you *have* bitcoins, there are tons of services that will imo be game changing in the porn markets if btc ever becomes big enough. Bitcoin allows truly viable microtransactions ($1-5) so paying an anonymous dollar to upgrade that video playing on pornhub to HD starts becoming an entirely new model people can exploit. Right now other sites have kind of paid lip service to these things, but they all require pre-existing accounts, registration, fraud control, etc.
Basically it needs to be as easy as going to a site, entering your credit card or paypal info, and having bitcoins 10 minutes later you can spend. Until then, or until massive bitcoin adoption, the mass adult market might be a tough one. Certainly solvable problems, but regulation will be the gating factor imo.
Also if adoption or ease of use starts to become viable, you are going to see all the marginal shitbox (and some downright criminal fraud) sites love bitcoin because they won't have to worry about chargebacks. If those types are allowed to go amuck and ruin the idea of paying for porn with bitcoin ("I paid .5btc and all I got was this shitty members area from 1998"), you might see a harder road to adoption until a big industry player starts using it with a successful implementation.
So yes, I think it could be revolutionary for adult site operators. Zero chargeback risk, very low cost for transactions, but probably won't replace credit cards for the traditional subscription based services. Webcam studios probably are already looking into it pretty hard, as it solves tons of issues there - it's simply a matter of how many of their customer based (whales especially) even know about it. Then you can go the PPV model or anything else you can think up
