Well, let's take care of some obvious things first. All of the characters I cited were actively being published in the 1970s and early 1980s. No surprise there, since that's when my childhood took place. All of the characters were also published by DC. That's not much of a surprise either, since that's primarily what I was exposed to back then. Most of what I knew back then came from a small collection given to me by an acquaintance of my folks, who happened to be fan of the DC characters in general. I don't think I even saw an issue of X-Men until I was 12 or 13.
In terms of visual themes running through the group, let me provide some summary info...
- 7 of the 7 characters wear gloves
- 6 of the 7 characters do not have capes
- 5 of the 7 characters do not have a centered chest emblem
- 5 of the 7 characters utilize the color yellow
- 5 of the 7 characters wear masks
I might also note that the costumes in question were designed by George Pérez (Cyborg), Paul Norris (Sandman, Aquaman), Dave Cockrum (Wildfire), Jerry Robinson (Robin), Carmine Infantino (Flash), and Gil Kane (Green Lantern).
Sadly, there don't seem to be any really cohesive themes here that I can see. Most of the categories I noted are fairly broad and could also include the likes of everyone from the Vision to Apache Chief. (Both of whom I was aware of, but had no real interest in.) The costumes original designs range over the course of decades, and have vastly different styles.
The closest thing I can suggest as a common thread is that these costumes are not especially "typical" for what you might think of as a superhero uniform. "Typical" being, I suppose, based somewhat on the Superman/Batman model. There's still the skin-tight spandex feel to them (even Cyborg) but they're executed in somewhat surprising ways.
"Where's Green Lantern's belt?"
"Are Flash's gloves supposed to be the same color as his sleeves?"
"Why is Robin's chest emblem off-center?"
Of course, the key is being able to make changes to the standard without going too far. As cool a character as Speedball is, and as much as I like Steve Ditko, that costume design was not attractive.
Of course, superior drawing skills help immensely too. The Blackjack costume I came up with in high school wasn't any good largely because I didn't have that much artistic talent to pull it off well. In the hands of someone like, say, Alex Ross, I'm sure it'd look great.
I suppose that, speaking visually, it's something that everyone is looking for. Your eyes look for the familiar and expected, but having something slightly off that expectation is a pleasant surprise. If it were totally expected and predictable, it'd be boring.
At least, that's my theory.