These are all valid. But I have seen some successful attempts at charging. It seems like identifying the right feature set to charge for is make-or-break.
Take Instapaper. The service helps you save and access content to consume when it’s convenient for you. Imagine you come across a fascinating article in The Atlantic and don’t want to forget to read it. You could email yourself the link or tweet it out, but saving it in your own personal Digg type of account is more natural.
Instapaper could charge for saving a certain number of items. It could charge for saving certain types of media. It could charge for exporting or sharing your links.
But instead, Marco, the creator of Instapaper, charges $4.99 for the iPhone app and variable amounts for a range of subscriptions. Of course.
Don’t limit use until you have some (sticky) critical mass. Capping the number of links would just make me not wanting to use the app for fear of hitting my limit.
Charge for luxuries — not the things that make your app functional. Technically I can still use the app. I can open up the links on the web and read. But it would be a luxury to have this content at my fingertips when I’m on a train or something.
Leverage the recurring subscription payment model. Nowadays, you don’t need to just charge $0.99 for your app. Subscription pricing allows you to charge a low fee for the app, but continue getting users to pay low but substantial (over time) fees that apply a meaningful multiplier to your lifetime value.
What are some other great examples of choosing the right angle for charging your users?