In case you haven't noticed, Power Man has changed a lot in the last few years. First off, he's no longer Power Man. He's just Luke Cage now. He also dropped his traditional Yellow-and-blue duds (which also included a tiara and a chain-link belt) for regular street clothes.
As I understand it, this change was done to make Cage more modern. To make him hipper and more in-line with current fashion trends. I don''t mind a costume update, but completely eliminating a costume all together does bother me.
My problem with the "street clothes" mindset is that the characters soon become indistinguishable from one another, especially for kids.
 There's a reason why there's a Green Ranger, a Blue Ranger and a Pink Ranger and not a Lt. Ranger, 2nd Lt. Ranger and a Specialist Ranger with the same costume.
Even if "New Avengers" — or even a Cage film — weren't marketed toward kids, you still need something — a logo, a hair cut, a color combination, some fancy jewelry — to make him look different from your average Joe. I would particularly hate it if he was simply marketed as "He's a superhero! ... and he's black!"
The old "chains, yellows and blues" aren't all that great for Cage now, but he should at least have a consistent look. Is he maintaining a consistent look in "New Avengers," or does he change his clothes from storyline to storyline?
I'm not a reader of "New Avengers," and his lack of a costume is one of my reasons for me not being a reader. For that matter, his (and Wolverine's) inclusion on the team is another reason I'm not a reader.
Why don't I like Cage as an Avenger? He isn't bringing anything to the table. Last I knew, he was mildly super strong, invulnerable and a good fighter. That combination ain't enough to be an Avenger in my book.