As much as I’m loathe to heap praise on Second Life & Linden Labs, I think they at least got this model right: they handled transactions between users that would be occurring anyway, and then allowed people to put money into the system & take it out.
When a user views a web page, a transaction implicitly occurs. Medium knows about both sides of this transaction. (With ads, the transaction is offloaded to one between two unrelated third parties.) It’s not unreasonable to make such a transaction explicit and broker it without third parties. If everybody starts off with enough credits to handle the whole lifetime of a very casual user, then the monetization will never affect normal users, which is a big hurdle.
Flattr is a lot like paypal donate buttons & patreon, in that it’s an add-on that individuals put on top of a service to provide monetization the service doesn’t provide by leaning on the good will of individuals. Such systems sometimes work well, but you have to lean pretty hard on the NPR model of begging for donations in order to use them.
If you already manage the content distribution system, allowing casual users to automatically pay money they didn’t know they had (with the knowledge that most of that will never fully circulate and that which does circulate can be more than made up for by minimum balances for cashing out & fees) lets you put the system in overnight without anybody noticing or caring & then only charge very heavy users. (If the costs involved are small enough — and if they are on the scale of ad revenue, they are tiny — heavy users will not feel like it’s a burden on them to continue using the platform.)