It's a question of first principles, and these are wholly independent of style or regionalism, precisely because they're universal --that is, equally valid in all places (and times).
1. intimate space
2. diversity through small-increment development by different owners
3. boldly-conceived infrastructure (Yeah, canals and landfill !)
4. buildings that touch
5. background buildings --if the paradigm is right.
6. roof forms and materials as unifiers
7. casually varied relationships between buildings (NOT defined by uniformizing rigidities of zoning)
8. small, irregular lots
9. a central focus or main square with a monument or two
10. architecture that's not hidebound with prissy strictures against frank revivalism ("We can't do that, it was done a hundred years ago.")
11. if the streetscape is sound, interesting and pleasant to look at, you don't need many trees. They take up room and divert from the task at hand
12 hundreds of small buildings give you more places than a few dozen big ones
13. if you build a great place you'll make money; you don't have to start with current market wisdom
14. make every square inch count
15 build in the hierachy; coherence will follow (put the most important things in the center)
16. bold topographic ideas like landfill and canals (you make the former with what you excavate to make the latter)
17. don't be afraid to design for the rich. The best things only the rich can afford (Back Bay, Beacon Hill --then and now. The rest of us visit to get our jollies.)
18. pint-sized streets:
19. an intimately-scaled water's edge
20. don't be afraid to design pretty, and don't design for your colleagues
21. don't be afraid to risk a little hokeyness (if you think about it, Back Bay had more than a little Disney in its genes)
22. Taste is perhaps debatable, but mediocrity can be legislated.