There seems to be a frequent assumption here that economics would always dictate a taller tower if it weren't for those damned NIMBYs. Also an assumption that the powers that be (in this case the BRA and Boston Properties) should have the postcard-view skyline crescendo somewhere on their priority list. The first proposition is demonstrably false, the second is just silly.
The tallest towers, and especially the tall, slim towers that seem to induce fellatio among some members of this forum, aren't often built purely on the basis of economics, they are built because the "statement" itself is part of the point. This was the case for both the Pru (it was seen by city leaders as an important affirmation of Boston's resurgence after several bleak decades) and the JHT (which Boston-based JH felt important to be taller than the Pru, a Newark interloper). It's also true of the "signature" towers in many second-tier cities and entire clusters in places like Dubai. That is why the Pru and JHT have been the tallest in Boston for so long - their justification was based partly on the PR impact. It's only been recently that the resurgence of interest in the city, particularly for residential, with fewer available parcels and higher psf returns has justified building taller - because the higher you go, the more you lose (on a percentage basis) to infrastructure (HVAC, core/elevators, etc). It's damned expensive. It's pointless to say, "well, gee, Comcast built a really tall building in Philadelphia, we should have one!" It's really, really doubtful this is on the priority list of Boston Properties, nor should it be.
Certainly the Seaport is capped by FAA. But I'm not convinced the economics elsewhere in the city yet support supertalls. The primary reason we get shorter and fatter is not NIMBY's or shadows, it is because generally more, shorter and squatter buildings maximize return. Especially for commercial footprints (residential favors smaller floorplans/more windows, much to the delight of the fanboys). We get plateaus not because the BRA is neglecting its postcard duty or "caving" to NIMBYs but rather because that's what the spreadsheets dictate, and developers are running similar spreadsheets based on similar inputs and assumptions! If you want tall buildings, instead of railing at the NIMBYs you need to convince someone like Ned Johnson that it would be a good idea to build a phallic monument to himself (doubtful).
In this case, as noted above, the fact that construction has to maneuver around rail and subway tracks and a highway also adds some complications, limiting flexibility of the podium and the envelope for building up. The success of this building will be based overwhelmingly on the skill with which the first 50-100 feet are designed (depending on your starting point, since some is below ground). Yes, I would be hugely disappointed if the design and materials were taken from the Kensington or its ilk, but let's focus on what matters. Here, more than almost anywhere ... given the transport hub ... the streetscape and pedestrian interaction should dominate concerns.
