If Boulder has a real visual flaw, it's the architecture -- away from the downtown/Pearl area, most businesses are examples of the worst in 1970s excess: faux chalets, faux pagodas, or bizarre Mothership creatures.
But along Pearl (which is where I spend most of my time, aside from my folks' suburban house), they've done an excellent job over the years preserving the character -- at a semi-educated guess, the majority of the buildings date to the late-19th/early-20th century (excepting the recently revitalized "East End," but even there, the buildings are largely tasteful and restrained) genesis of the city. I can't seem to find a good book on Boulder's historical buildings, but the number of prominent 1880s and 1890s dates on storefronts is gratifying. (Not that I exactly have an expert, unfailing eye: yesterday as I wandered around, I looked lovingly on another wonderful example of old buildings -- then noticed the enormous "ERECTED 1998" on the front. Whoops.) Living now in a city that's largely neglected its old buildings until it's forced to tear them down, that's pleasant to see.
It's sometimes taken to a ridiculous extent, I guess (as is the case with much of Boulder's good intentions). The papers last weekend were filled with some discussion of 1950s tract homes being designated "historical." But in many cases it's probably a good barrier against the worst instincts of developers. Down at Pearl's West End yesterday, there was a bunch of construction in the "unprotected" areas, developments that were looking soulless and dull (or strange: why anyone outside Arizona would want to emulate the adobe-lookalike styles of suburban Tucson is beyond me), and in the midst of it, the lonely little building to the left -- boarded up, surrounded by mud, but protected by a "landmark" designation (half-assed research didn't tell me what it was, but it's a landmark). It had more character than any of the buildings in the vicinity, and thanks to some actual foresight, it'll stick around for a while, eventually getting reused in some way or another. Downtown Boulder's track record in these matters is hardly perfect, and people involved with the preservation would likely squawk at any praise for how the city's treated its history, but compared to many other places -- it could be so, so much worse.