Hey Shack,
Total noob here, so forgive any protocol faux pas...
I had headshots taken 3 years ago, so I can relate my experience from the other side of the camera:
- When my shots were taken, it was still OK to shoot B&W, only because color reproduction was less perfected and pricey (color was pretty much becoming the norm in LA for film/TV, but B&W was OK for NYC because of live theatre traditions). Today, color is IT for new shots, only people working from older shots can get away with submitting B&W. My photog shot RAW so he could produce either. I've got the B&W version, but have to switch over to color when my latest batch of repros runs out.
- Your thoughts on composition sound good - some shooters who've mastered the lighting and composition requirements do sometimes use unique backgrounds (brick walls, rooftops, streets), but done badly (and I've seen some of those) it becomes distracting and detracts from the purpose of the shot, which is to make the subject look their best. (There's nothing like seeing a streetlamp sticking out of someone's head like an antler; LOL) There are probably photogs who break the "patterned shirt" rule, too.
- More on that point; some subjects (and photogs) will overdue makeup/lighting/post-production looking for a perfect portrait, but casting people HATE when the h/s is so overproduced that it doesn't look like the subject anymore. Auditions are given based on the face in the picture, and if that person doesn't walk through the door, the casting director will be pissed at their wasted time, will likely reject the actor no matter how good they might be, and WILL remember them in the future. The best description of a h/s I've heard is, it should look like the subject on their BEST day.
- Don't know about proper aperture, but make sure entire subject falls into DOF (no fuzzy nose-tips or ears). And my photog bounced light up under my chin & nose to eliminate those shadowy areas.
- I've seen hundreds of h/s, and the most important feature to capture is likely the eyes. In the best shots, there is a spark of life in the eyes that makes you linger on the picture and piques your interest in the person pictured. In the ho-hum ones, they eyes are dead, and the picture is not engaging, no matter how beautiful the subject may be.
In NYC and LA, the most prestigious reproduction shop (and probably the most pricey) is called simply "Reproductions". Go to reproductions.com, they have galleries of samples from scores of pros in those cities. See what you future peers are doing. If you're in/near NYC or LA, they also keep more extensive books of samples that anyone can come and browse.
Some shooters are excellent generalists, while others seem to excel with certain subject types (women, model-types, "regular" people, etc). As a "regular" guy, I looked for someone whose book had good shots of my type. As you build your portfolio, you may see a pattern emerge from your best work; could help you target your marketing and build a niche for yourself.
Sorry for being long-winded, but I hope that helps a little.
Good Luck!