I'm definitely a bit of a curmudgeon when it comes to email. Email is about communication and html gives people tools that they mostly use to make communication harder, not easier. For me, I prefer plain text email when I can get it. The main issue for me is that most of the uses of html email I see simply do not increase the utility of the email from the reader's perspective.

Large signatures don't communicate anything useful to me that isn't in the email header. The phone number can be nice, but you didn't need html for that. Your brand identity that takes up half a page (more since I keep email in a small window to the side of my monitor) does nothing other than obscure the content of the email. Trust me, sending me an intensely formatted email with lots of pictures does not increase the likelihood of me doing business with you.

99% of email I receive contains < 1 paragraph of useful information. I would prefer if people just sent that. Then I can read it quickly (since it uses the same font as everything else, not that faux handwriting font you found somewhere and thought looked great) and respond quickly and helpfully, which I hope was the purpose you sent the email in the first place.

There's probably a 1% where a nicely formatted document with graphs and images and headers are helpful. But you can just attach a document to the email if you're going to put that kind of effort it.