I'm more bothered by her age, honestly. Then again, I understand that there have been other teen heroes before. I don't care what kind of a genius she is, she is still a teenager with a teenager's malformed, immature brain that leads to certain thoughts and feelings. If I were a comics reader I wouldn't find it appealing on those grounds alone. Either they're going to put too much teen angst onto her, which will be a turnoff for readers that are used to Tony, or they will ignore that and make her too adult-like, which seems unrealistic.
I also wonder if they can resist the urge to sexualize her.
All the other issues aside, I think it's certainly easier to do these replacements with heroes that don't actually rely on some super power. Tony Stark built himself an Iron Man suit; he's also made similar suits for others. Bruce Wayne has all of his fancy rich boy toys. But many of the other ones have unique powers that can't be passed down, like the Hulk, or those of the Fantastic Four, so I'm less okay with it in that regard.
What also annoys me is that, if these new comics don't do well, they will put it down to it being a female, or a black female, or just a young person, when in the majority of cases people just want to see Tony Stark as Iron Man, the one they've been reading and getting to know for however long.
It also seems shallow in that, most of the time, these are temporary replacements. One of the more common refrains I heard for the new female Thor was, "Don't worry. Dude Thor will eventually be worthy again and Mjolnir will chose him again, as has happened in the past; we've been here before." Thor was apparently also a frog at one point. However, I will say that I thought the passing of the torch to the new Spider-Man was done pretty well, and that seems permanent (in those comics, anyway).