1) pattern matching on the shoulder and gauntlet;
Essential.
2) side gussets
Nice, but unecessary if the sewing is good.
3) horizontal buttonhole on the sleeve plaquet (why must it be horizontal?)
A) Easier to button and unbutton; B) matches the direction of the cuff buttonhole; C) keeps the gauntlet from sliding back and forth along the buttonhole, thus properly closed.
4) yoke (split or not)?
Split. Most bespoke shirtmakers will do this as a matter of course, whether your body really needs it or not. One nice thing: with stripes and checks, the yoke can be "chevroned" which effects that attractive angle pattern in the yoke. I like that.
5) spread collar with zero tie space
Personal preference. I prefer no tie space. But if you like really large tie knots. you might want a little.
6) side pleats or plain back
This is both a matter of fit and the preference of the shirtmaker. I have had shirtmakers tell me that I absolutely need side pleats, and others tell me they are uncessary for my build. I let them do what they want. I do gather that it is harder to make a shirt with no pleats in back if the client has a muscular upper body or "salient blades." But it can be done. If you do get pleats, remember that like any pleats they should remain closed when not under stress. Check that at the first fitting.
7) MOP buttons
Essential. Nice big thick ones, with rounded edges and convex underside.

regular tail or square off as with Charvet shirts
Personal preference.
9) stitching on the collar
The standard is a quarter inch from the collar edge (same for the cuffs). Edge stitching most often looks flashy. I only like Kabbaz's version.
10) what does havinng shanked buttons mean?
A short stem of thread separates the button from the placket. No shank means that the button is sewn on flush to the placket. These are harder to button and unbutton, and also cause the plackets to pull and the buttonholes to gape.
11) back of collar height
Totally depends on how you are built. Higher is better, but your neck length determines how high.
12) interlining of collar and cuffs
Soft for cuffs, stiffer and (perhaps) fused for collars. No fusing on cuffs. Not for me, anyway.
13) cuff height
Depends on your build and personal preference. The bigger you are, the bigger they should be. I get mine fairly wide (meaning from top to bottom) because I like the look.
Are there any other stylistic items that one should consider?
Pocket or no? (I prefer no). French front or two plackets? Armhole height? Closeness to the body? Length of collar points? Degree of spread? Shape of collar leaf? Length of shirt body? Monogram?