This is my point - it's not a problem for people who are living and breathing SSL certificates, but for people who don't this is a complete mystery as to what the next steps are.
Please think about who uses Let's Encrypt - it's everybody from seasoned experts through to novices. You need to cater for all levels.
I appreciate some clients will have too many domains to list, but why not provide a way for us to at least see a list somewhere, or truncate the list in the e-mail so it gives us a little bit of a head start.