I disagree completely. One, let's look at all the heroes people read about who are humans who routinely take on superpowered beings. I don't know if you guys have heard about this little character called Batman, but he's been fairly popular for the past 80 years or so, and nobody seems to want him to have superpowers.
Look at Whendons comments about this show. Someone posted them in the General Discussion thread. This show is purposefully about people WITHOUT powers. That's the focus of this show. Now, this show already is consistently in the world of super-powered beings. We're going to see a lot of them in this show. But I highly doubt we'll see any on the team. We'll most likely see lots of super powered enemies, and probably even have a super powered friend who guest stars and helps out in an episode.
But the main characters are going to be the normal humans. Because that's the purpose of this show. To show how normal people can still be heroes in this insane world.