I don't believe DL's strength in Boston has anything to do with the lack of success on the LGW run. In my opinion, pure and simple, it was the route that was the dud...not the carrier, not the respective cities. People just think 'Heathrow' when they think of London, and when there are so many LHR-BOS flights (and most, if not all, do well), LGW is easily pushed into the background. Virgin made a go of the route because, well, they are Virgin. There's a following with that carrier wherever they go. And the fact that DL could not make a go of this route even after Virgin switched to Heathrow is even more telling: people want Heathrow.
Now, what does this say about other transatlantic routes from Boston? On the whole, you've got to expect that, by and large, you're not going to fill planes all the time in the off-season. There are routes that DL's 767-300 might have been better positioned on...Dublin, for one.
With so many other ways to London from most cities, the DL BOS-LGW route was easily lost in the noise. I hope they don't give up transatlantic plans for Boston, just think smarter about a better airport than LGW...especially when there are 7x BOS-LHR flights to compete agaist each day.