The problem with him being silent(at least to me) is that he has a rigidly defined role and a name as well as backstory. We even know he is from another island and what he looks like. Who he cares for. What interactions he has had in the past. People talk directly to him and then there is just this awkward silence. It would be one thing if he never had a name and you could cast yourself into the character but he does. He is a character. He has perceived reactions. You could define who he was by the myriad choices they give you. With varying ways of defining his personality throughout the game. You have Corvo as a nonlethal character? His dialogue vaguely reflects that. A bloodthirsty ghost of revenge? Dialogue for that. This would make the game come more alive imo. It is fine how it is. But it would be a better experience as well. Again my opinion.
I am all for a silent protagonist, where it is appropriate. Like Dark Souls, TES, Fallout, Etc etc. Where the protagonist has no or miniscule defining background. Corvo could have been referenced as Guardian/Protector throughout the game as opposed to his actual name and then we would not have this issue. I am not advocating a loud mouthed Corvo. Merely a gruff, not very talkatve protagonist. One who only really speaks when absolutely necessary. It would have added much to the game imo.
I have to disagree, I don't feel as if there's something missing unless it's a game that constantly shows you the protagonist's appearance but doesn't give them a voice (Fable 1 and I believe 2).
I feel as if it takes a little imagination to immerse yourself well with a silent protagonist, you have to imagine what they would say when spoken to directly and such to not feel like you're missing something. Better than them just using gestures in conversations (like in Fable, great game but seriously why would they think that was a good idea?).
That's your opinion of appropriate. I personally have ended up disliking or feeling confused by voiced characters while in first person mode. It just throws me off that somebody who you are not physically seeing on the screen is talking, unless it's in third person where you can see him/her. I feel like it would've taken away far more than it added but if you're a Crysis fan or a Bioshock fan (such as I am) then good news, both have first person protagonists who speak (I won't know how I feel about it for them until I play them).