I think your B&W treatment lacks of contrast : it's a bit "greyed" (not sure of the word... if it does not exist, then I just created it ). Your black values are not really black, and same for the white values.
Concerning the composition, I'd say that the main subject is not so evident. I mean I don't see what you want to show. The structure of the roof makes me think that you wanted something graphic, but there are too few graphic elements to give that effect. (But this is only my point)
I agree it needs more contrast, but if they added any the bottom half wouldn't be visible again.
I have no idea what I'm looking at when I look at this shot. Is it the crowd? If so what part of the crowd? Right now all I'm looking at is that huge thing blocking the arch of windows. Try a different angle next time.