Different reading habits... asks for different layout options.
I have offered my readers the option to choose whether or not they want to see a preview snippet (description) for each headline in their newsletter.

What options do you offer your readers to "personalize" their reading experience?
I've seen that a number of readers subscribe to newsletters through RSS feeds or third party services that strip the formatting. Do you think users lose value from your newsletter if they strip out the content and remove all formatting?
When content is changed, yes. When formatting is changed, then no.
Looking at how I consume different sources, I rather have them all in a format and layout that I prefer. Then I know it is easier to read and share.
In this case it is purely the content of the newsletter as compiled differently for 2 groups in MailChimp.
I started doing this when I got some (literally 2 readers) backlash when I shifted from headlines only to headlines with summary.
Some people prefer to see the daily headlines in 1 view... others appreciate the occasional scroll and a preview.
It made me think that besides the content options (which topics) that some newsletters offer, layout options should also be included. So far with positive response and an about 70-30 division for the 2 layouts.