But certainly Toby, it is possible to create a functioning underground space. The ones in Boston you mentioned all seemed to be afterthoughts or accidents of underground spaces...not really intended for people to populate regularly (Green Line tunnels). Montreal is simply an issue of awful architecture, unique only due to the fact that it is belowground, and if I can remember well enough, lacking in light.
Since CHP is on a grade, it would really only be halfway underground. By taking into account the same principles an architect would use to design an outdoor market, you would ensure uniqueness and quality of space, and with a little effort, would have light sufficient enough to make you feel like you weren't really belowground (or more like it, in City Hall's cellar).