In general I agree with much of LM's assessment, and it was pretty much how I felt when I first moved down here. But beyond the similarities in urban fabric or the stats, at several levels Boston is still very different from Toronto - while it has a roughly comparable metro area and pop as Toronto, it has a much denser core (Boston, Brookline, Cambridge, Somerville) but much less dense suburbs/exurbs. It has a fairly cosmopolitan population (by American standards) thanks to its large number of students and researchers, yet most of those are relatively transient populations that usually don't stay for much longer than 5 years. Boston is both smaller (size, multiculturalism, importance as a financial centre) and bigger (history, impact on the world's politics and sci-tech) than Toronto.
In downtown Boston you do *not* see the kind of shambolic public spaces and streetscapes we are so inured to in T-O, and that's because the locals would never tolerate them.
While Boston does have a penchant for brick pavements that usually look "nice", and they generally don't tend to tear up pavements and just patch back over with asphalt, there are a lot of places with bricks gone missing or broken and never get replaced. There are also a dismayingly large number of dead planters and trees. Sure, Boston doesn't have utility poles in the core, but that's one relatively minor thing, in my opinion. There're still plenty of posters and graffiti, not in the main tourist areas or prime residential neighbourhoods, but certainly in the more bohemian areas just outside.
I think the answer in Boston's case is 'not much.' There's just very little physical scope for renewal in a city that's already so built out. I could count the number of major construction projects I saw underway on my last visit on about half of one hand.
More importantly, I think Boston shares with San Francisco a certain small-c conservatism in matters of design, born of an understandable reluctance to mar what are very attractive historic aesthetics. This is one way in which Toronto's famous lack of a cohesive style or identity can be a great boon; we take for granted just how open we are to experimentation.
As adma pointed out, Boston certainly had a rather sombre history of tearing up entire historic neighbourhoods (Scollay Square, West End) in the name of urban renewal, but that is in the past.
In terms of current construction project, the recession did hit pretty hard, but barring any major catastrophes a good number are actually restarting / will restart in the next half a year or so. There are actually several large swathes of the city at the outer edge of "downtown" that are currently / soon undergoing major renewal: Fenway (used to be a sea of parking lots and auto shops, has seen two projects in the last decade, with 2-3 more starting soon and more in the plans); NorthPoint (huge development across the river from downtown, former railyards, a couple of buildings completed but stalled for a few years, relaunched a couple weeks ago); South Boston Waterfront (former portlands, by far the largest carte blanche that will be filled out over decades, several buildings completed and a couple more just started construction); lots reclaimed with the Big Dig (a couple completed, another 2-3 just started construction, but two of the biggest ones, replacing current parking garages, are mired in design/regulatory quagmire). Then there are the air-rights parcels above the Mass Turnpike scar cutting through the city: one high-profile dev died a painful death, after construction started, due to financial and legal issues and corruption charges, etc, and another one just starting but also being sued for various reasons; but a package of parcels in Back Bay is now kicking into action, and densification of some already-built parcels are also moving forward. Various lots in downtown that stalled in the last few years (similar situation as 1BE) have just resumed, except for one prominent one right in the heart of downtown that is still literally a hole in the ground. And of course there are all the universities, colleges, museums etc, which have mostly gone ahead with new construction unabated throughout the recession.
Bottomline is, Boston is still very much changing and developing, though unlike Toronto's highrise boom, the majority of new developments, including most of the ones mentioned above, are midrise (10-15 stories), both to conform to the city's existing built form, and to avoid confrontation with powerful NIMBY groups that cry bloody hell over shadows (a favourite tactic, so much so that the state legislature is currently considering legislation that wants to bar any new development from casting a shadow, at any time of the day/year, on Boston's main parks/squares).
