When I buy shirts, I look for:
- The right collar
- The right fit
- A fabric that I like
- The amount of handwork that goes into the shirt's production
For dress shirts, I prefer the cutaway collar. It dates back at least to the middle part of the last century and was, and still is, often worn by the British aristocracy. They were the style icons at the time. I have not found another collar that come close. But that's just my taste.
Personally, I don't care for too much "guts" in the collar. A softer collar with the option of using collar stays for me is less rigorous and gives an appearance of confidence without "trying too hard". In a sport shirt the collar is less important, but we still use the cutaway a lot.
As to the fit, the length of the shirt is important. When you lean over to tie your shoes, you don't want the shirt to pull out of your pants. Some people wear their shirts outside their pants and therefore want a shorter shirt. I'm OK with that, but don't compromise. Decide when you buy the shirt whether it's a dress or a sport shirt. The longer the tail (of course up to a certain point), the less chance you have to develop a blousy look. The shirt should be fitted, but not so tight as to outline rolls, love-handles or have the chest buttons pull. The shirt sleeves should be long enough to show under a jacket. If a little too long, move the button over to prevent too big a cuff opening. Uncool to have the cuff approach the knuckles.
The fabric must be pleasing and needs to fit into my wardrobe; solids with busy coat patterns and stripes or checks with solids or more subdued patterns like herring-bone. I like crisp fresh colors, forget dusty, and I look for contrast. Can't go with grey shirts and grey suits. Makes your skin color look even more grey in the winter time. Never understood the tonal look. Makes me think of the commuter trains.
The amount of handwork that goes into the make, translates into how well the shirt moves with the body's motions. The more handwork, the better it moves, but at the same time, the more expensive it is. Let you budget dictate.