My concern is primarily that Evans can't act. I don't know this for certain, however, since I've only seen him in the FF films and, let's face it, those aren't exactly vehicles for actors anyway. I mean, I think both Michael Chiklis and Ioan Gruffud can act quite well. But you wouldn't know it to look at that movie.
Basically, I think the success of a Cap movie will come down to three obvious things: writing, direction, and acting. I tend to think that good writing and good direction can make up for middle-of-the-road acting. Evans may be a middling actor who can rise to the challenge with good material and a good director.
My concern, however, is that the casting of Evans is evident of studio meddling, which suggests that they will hijack the film in favor of what is "marketable" or "bankable." So, Evans will end up being saddled with a similar approach to how he played Johnny Storm in the FF movies, and will be cracking wise as Cap. WRONG for the character. Some humor is fine, but I tend to see Cap as far more earnest and patriotic in a serious way than sort of casually goofy. Cap isn't a demigod like Superman, but he's got a lot of the same core values.
A good Cap movie will recognize that Steve Rogers is -- at his core -- a patriot. He's willing to subject himself to incredibly dangerous (but ultimately successful) medical experiments. He's ready to place himself in harms' way in a VERY visible fashion, because he recognizes the need to be a living symbol and inspiration to others, and because he so fervently believes in his mission. After he's frozen, and wakes up in a world he barely recognizes, he recognizes that he still needs to play the part he once did -- perhaps more than ever, even though he might be far more conflicted about the increasing presence of "gray areas" in modern society and in his mission.
My concerns are (A) that Evans can't really play that kind of role well, and/or (B) the studio is totally uninterested in that. They want a rock 'em, sock 'em action film set in WWII, but they don't give a damn about maintaining the feel of the era. They'd rather it be "hip" for modern audiences, chock full of lame humor, etc.
That said, when I saw the first production stills for the original X-men movie, I was TOTALLY unimpressed. After walking out of a matinee showing of the film, I was delighted with the film. So, hopefully, they'll surprise me. I'm wary, though.