Honestly, it's the team. The city (night life, racial history, critical media, unglamorous, etc.) probably factors in, but plenty of free agents have come here. Pietrus seems to be having a great time.
The fact is that we're old and our players have gotten a mean rep. Howard for example, is a fun-loving Big Baby type. KG made Big Baby cry...At the all-star games the Celts have been reported to isolate themselves and talk trash.
Our team mentality is that of the Kobe generation (win by any means, opponents are enemies, don't fraternize with enemies). The current crop of young talent has the mentality of the Lebron generation (at the end of the day it's just a job, opponents are friends, have fun).
Our future is unknown. Rondo is a stud but what other young talent do we have? I don't want the Big 3 to go but eventually they're going to be more $ than they are worth. And Rondo doesn't have a bunch of NBA buddies, he's old school. I don't think top FAs see long term potential AND a friendly environment.