Unless you've run a marathon in temps over 65-70 degrees I don't think you can understand the affect it can have on your race. The weather on marathon day affects you soooo much more than any track or xc race, so while we may not say 70 degrees is "hot" on the roads in a marathon it feels like 90.
You combine that with the fact that Boston fall right when most people have only hand a few weeks of Spring like weather to train in, well the results speak for themselves.
For me heat is my Kryptonite, no way I would expect to run close to normal in yesterday's conditions. Ideally I'd want temps in the 40s-mid50's with cloud cover.
I have run a marathon in 60-70 degrees, which is why I would go out more conservatively with the forecast they had in Boston. Next year I'll be heat acclimating in my prep.