I believe that it's the options a city has that define how good their nightlife is. Boston has options. There's enough for the live music crowd, there's an OK club scene, the Pubs are fantastic (better here than any U.S. city i have ever visited), the window shopping is nice, and to top it off, there are some moderately shady areas, but I would say on the whole, Boston is pretty pedestrian friendly even at night. I feel safer walking around drunk after an evening here than i do even in Providence, RI (Baltimore, MD is also scary at night). The variety makes it obvious why Boston has good nightlife.
I took it for granted until i spent 2 years in and around Portland, ME. Portland's nightlife is awful in terms of variety. You can go to the "Old Port" which has a couple of pubs, some have DJ's (Cake, OPT, Liquid Blue, 51 Wharf), and some have terrible to moderately bad live music (Gritty McDuff's and Bulfeeney's). There are also places like the Asylum which host small concerts and events (i went to the Q-Ball there at Halloween this year) but there are no dance clubs, no places to see good music regularly, and everything closes a full hour earlier at 1am. I know Portland is a tiny city and it's a great little town (despite my using it as comparison), but the lack of options kills me.
Last year, for my girlfriend's birthday she wanted to get dressed up and go somewhere nice. Since we were visiting her family in Scarborough, ME, we had to go out in Portland. Well, there are some OK restaurants, but going out for drinks after dinner dressed in a sport coat and jeans, I automatically felt over-dressed for any venue. Experiences like this have made me miss Boston when I'm away. The beauty of this city is that there are plenty of options for whatever you feel like. There's something to the 50 something artsy crew, or the frat-boy college kid.
There is room for improvement. Most notably in my mind, i wish the trains ran even an hour later. I can't see how cutting off mass transit 1/2 hour or so before the bars close makes any sense...talk about encouraging people to drive drunk. I would also like to see some more late-night food staples; places like Via Via in Newport, RI and Golden Crust in Prov. stand out in my mind as favorites, but even a few IHOPs or Bickford's would be nice. Maybe an hour later for last call? In the end, Boston's a fun town and being away made me realize that i took it for granted. The people who say Boston "sucks" for nightlife obviously haven't been elsewhere or have not really explored their options here.