The architecture is something I really pay attention to in all of the games I play, and yes Diablo has a quite impressive architectural potential. I hope that the team will reintroduce some of that nice architecture in Diablo 3.
Although when I look at the world map I fear that the majority of the places ( correct me if I'm wrong I haven't read the books) won't have a Gothic architecture. The variety won't hurt for sure, but I'm afraid the Gothic theme will get lost instead of being the dominant theme.
What i really wouldn't like is for them to introduce an elfish architecture/region...elfs remind me of Warcraft and i wouldn't like to see Warcraft and Diablo connected, that would just kill the Diablo atmosphere. Altough it might be possible to make some dark and less cartoony elfs than in warcraft, this still scares me.