Subject lines are hard. Your subject line can fail if it is:

1. Not descriptive enough. Tell me what the email is about, I need to know whether to read it, file it or delete it without reading the body.
2. Too long. I need to be able to read the subject on my mobile device and gmail cuts subjects after about 12 words. Gmail is just being generous though, 12 words is far too long. I’d suggest less than 6.
3. Too short. One word is not (usually) going to be descriptive enough.
4. Full of “Re:” and/or “Fwd:” more than one of either is too much, they waste valuable character real estate.

As a rule of thumb, try to use 3 – 6 descriptive words. An extra word or two –  surrounded by separator like a pair of braces – for a tag is also acceptable if you need that for your GTD filters.