I wonder why they don't collect email addresses for people using their service. It would open all sorts of opportunities for marketing, soliciting donations, and also service announcements. I mean, I heard about this campaign on Hacker News, not directly from Let's Encrypt.
Also, I'd probably sign up for a small recurring donation if it was possible. Recurring donations could become a significant and reliable source of funding.
Edit: I am not suggesting they start spamming everyone and ask for money. An opt-in email list for topics not strictly related to service would be good.