https://github.com/chromium/ct-policy/blob/master/ct_policy....
You may find this useful: http://www.certificate-transparency.org/how-ct-works
Whether you use any specific CA, like LE, or not, has no security impact.
It's about what your users trust and you don't control that.
To be perfectly honest, I don't really trust the other certs, either. I mean, I pretty much have to, and having a mainstream CA sign a cert does provide a bit of reassurance -- but only a bit. I don't really consider CA signing to mean that the cert is "trustworthy", because I don't really trust those CAs, so if they're the anchor for a chain of trust, then the chain of trust is weak.
If you can receive a http request destined to the target domain (e.g. via MITM near the real target, DNS hijacking, or route hijacking, or MITM near a CA) then you can get a cert issued for that domain by pretty much any popular CA.
With security so limited what would be the purpose of compromising lets encrypt?
Like any other CA, they do have the technical ability to sign arbitrary other certs, so could issue a cert for MITM. As some other comments show, certificate transparency is starting to reduce this risk.
