Apologies, I'd not meant to appear thick. I'm just suggesting that there need not be a distinction. Pricing innovations in the desktop space can also apply to the web space.
Subscription means you have to convince the customer that enough value is being delivered consistently to make them feel comfortable paying month after month automatically. Single purchase means you only have to convince a customer that value is being delivered right now. The single purchase decision is a much lower decision hurdle.
In the real world, single purchase decisions vastly outweigh subscription purchases in both value and volume. So why do web apps, lemming like, aim at the higher hurdle? As the responses to your question suggest, unless you are netflix that is usually insurmountable. Even well proven physical subscription models (newspapers, magazines) are having a hard time making the subscription model work on-line. Hence the fallback to advertising revenue or hoping to be bought out.
If you can't give the customer something real, tangible and permanent now then you automatically assign yourself the tougher challenge of convincing them that the site will still be around, functioning and delivering value into the future.
Example: $20 gets you into our web app. All your needs met and delivered to you in Excel/OpenOffice/GoogleDocs format...whatever. This is yours to keep forever and you get one year free access to our fantastic service and all its upgrades.
I've a few more specific ideas, but I'm saving them for the day I decide to ditch my life as a well paid corporate drone.
Wait - just thought of another one. Its not on my list, suffers from a need to get physical and has probably already been done, but illustrates that a single purchase, subscription-like model is possible in theory.
webPrint.now Dump a digital image on webPrint.now and we'll get a high quality physical print to you overnight. A gazillion file formats supported. Sizes from tiny to gigantic. [Single purchases, but repeat customers will be the best business.]