Other than that it's great.
I mean this both straight and a bit sarcastically. I'm ideologically inclined to want to see it work. But it doesn't, it hasn't, and I see little reason to expect that to change in the future. Patreon doesn't work perfectly, and perhaps there's an even better model waiting for someone to find it, but it does work.
It does strike me as the sort of thing that would be a decent VC candidate. It is plausible to me that Flattr's core problem is an activation-energy one. But I can't run that experiment myself.
Where does patreon fail, aside from adoption and popularity?
I don't think it's a transaction cost issue; Patreon charges my credit card once per month (aggregating multiple $1-3/mo charges into a single charge), and I believe they pay the artists once per month too (aggregating hundreds or thousands of contributions).
So... the flattr model :/
I tend to agree with the comments above that the Flattr model doesn't work, but I think it would certainly be ideal for a lot of people creating content on the side if it did.
I agree with your main point, but to clarify: Flattr was specifically designed for this sort of one-off usage. Clicking a Flattr button doesn't subscribe to anything, it just adds that person/site to a list on your Flattr account. At the end of the month, your money is divided evenly between those on the list, then the list is cleared.
You can set up subscriptions on Flattr, which sounds more like Patreon's model, but the default mode is for one-off drive-by donations. Flattr subscriptions are just an automated way to click a Flattr button once a month.