Seamus, I feel like you just said that there is no difference between Quincy and Cambridge, and then listed the differences between Quincy and Cambridge. Cambridge has better proximity to the parts of Boston that people want to be close to. In addition, Cambridge is itself a generator rather than just a taker. The same cannot be said for Quincy - certainly not to the same degree.
As for Quincy being "so much better connected to the city than most or all of Boston's neighbors to the north and west aside from Cambridge." Hmm. Quincy is connected to outer parts of Boston, so I might equate Quincy to Revere, but Quincy is certainly not as "connected" to the city as Cambridge or Somerville. While bordering Dorchester means that Quincy borders a place with a Boston zip code, it does not mean that this proximity is as valuable an asset as greater access to the Back Bay, DTC, Waterfront, etc. That is, technically bordering Boston does not make Quincy any closer to where you want to be in Boston than Medford or Malden, which have as good or better access to DTC and immediately surrounding areas than Quincy.
If you're not generating value (the way that Cambridge does, separately and in addition the benefit it receives from its proximity to Boston) what matters is what part of the city you are close to. Parts south were annexed whereas parts north, for the most part, were not. Refer to the Ferdinand thread where Dudley is identified as the geographic center of Boston. With that in mind, consider that if you live just north of Boston, you are closer to the South End than someone living just south of Boston. That is, people living just north of Boston are closer to all of the places in Boston that make Boston desirable - the places and institutions that people want to get to.
Finally, I would argue better public transit to the north. As for driving, consider that you hit the Boston line just before coming over the Zakim bridge going south. Now consider driving north on 93 from the South. You leave Quincy and still have a quite long way to go before you're "in the city."