Money is the #1 factor. We've been pretty tight to the cap for the past 6-7 years, so obviously we won't get a lot of big fish because we don't have the cap space. So yeah, right off the bat, that's pretty undesirable.
You saw how Backes said a huge motivation to come to Boston (besides the aforementioned 6m a year) was the chance to play with guys like Bergeron and Marchand. Bergeron has gotten a lot of praise from pretty much every fanbase, but I'm sure he's even more well-regarded around the league.
Boston is also a huge hockey market. The fans are extremely passionate, that's a good thing if you're a good player, a very bad thing if you're not. Hard working guys like Backes see that as motivation. A player like Bergeron can do no wrong here, he very rarely gets any criticism because he's been the standard of consistency and excellence for years. It's not easy to reach that level, but it's extremely rewarding when you do. I'm sure a lot of players would prefer being a star player in Boston/Chicago/Toronto/Montreal/other huge markets over smaller markets like Florida. So the market size really depends on the player. For guys like Beleskey and Backes, that was a big factor.
The weather here sucks, yeah, especially in the winter. But the majority of the players in the NHL are from Canada, countries that are as cold/colder than MA (Russia, Sweden, etc) or states that are as cold/colder than MA (NY, MI, MN, etc), so I doubt that's a really big factor. A lot of states have much better tax benefits, though.
I don't think Boston is any less attractive than any other team. If you're raising a family, the suburbs outside of the city are beautiful and then education is top notch. If you're a party animal, it's the college capital of America, if not the world. If you're neither, it's a cool city. Not as big as Chicago/NY/etc, but it's still a major US city with loads of history behind it.