http://www.quora.com/What-are-the-pros-cons-of-webgl
For those burnt in the past by VRML and propietary 3D web plugins (Adobe Atmosophere, Google Lively et al), this is a good place to start.
One question nearly keeping me up at night (but overshadowed by the unpredictable demands of a two year old) is how do you get around using a technology to design 3D virtual places despite the technology (html 5) not having a physics engine? For virtual heritage I think it is it an interesting question and it can be answered.