I guess I'm an amateur when it comes to this, but I just have to ask: what about CAcert?
I came across it a while ago for a different reason, and found out that they too issue free certificates, provided you prove your control of the domain. I know that the reason nobody uses it is obvious; they don't have their root certificate in any major browser yet. However, if everyone just ignores the free CAs that don't have their certificate installed, then won't everyone who wants free certificates be forced to use LetsEncrypt?
It seems odd that, in response to big, corporate CAs, we're going to use one singular free CA. It guess it frustrates me a little that if I want a free (usable) certificate, I have no choice on where to get it. Anyway, does anyone know why I can't seem to find very many people advocating for additional free CAs?