Return Path analyzed two years of email marketing data and found that the subject line length drastically affects response.  Their study found that click-through rates were 75% higher when subject lines were 49 characters or fewer, as opposed to 50 characters or more (spaces included).  Open rates were 12.5% higher within the same parameters.

Of course, accounting for the actual text of the subject line is a different matter.  Well-written headlines or, in this case, subject lines can make all the difference.  But it is well worth noting that keeping it brief can seriously affect the bottom line for email marketers.