For me the wordcount is one of the most relevant infos and I love newletters that provide it. After fandom, pairing and author, length is the major factor for me to decide to click. I almost never click for drabbles or much of anything below 1000 words, but I will almost always check out new longfic. Obviously YMMV, but for me it's the most relevant.

I appreciate all the header information, warnings, tags and such if authors give them, but I don't look for it to a newsletter to provide everything, just enough to entice to click and then check the header.