martinibuster - 3:28 pm on Dec 23, 2012 (gmt 0)
This is like a prime fishing spot a mile or two hike from known fishing spots. Anglers don't publish specific spots with directions on how to get there, including landmarks for identifying the exact spot. It's hard won information. Even books on where to angle for specific fish stop short of naming specific spots. Once that information gets out and the hordes descend upon the fishing spot is called burned.
A specific subject line is like a prime fishing spot. It's hard won information. If it is particularly efficient it can be considered an ingredient of your secret sauce, your mojo, so to speak.
General characteristics of a subject line is an interesting topic that won't burn subject lines. Capturing the attention of a link prospect is a good starting point. The opposite, being boring is another one. Being similar to other link building messages is another not-good attribute.