I'll still tune in to every Celtics game I can, but agreed I just don't watch many other games now. Part of it is that I have a busy life, but I have free time I spend on other things. I think the reason is two fold -
The televised games are typically a lot of the teams with big name players, and so there is terrible officiating. The officiating in the league bothers me so much that people around me tell me to shutup and stop talking about it during the games. I harp on it, and it ruins the games for me.
The other thing, and this has ruined most sports for me not just the NBA, are the incessant TV commercials and aggressive advertising. I am one of those people that will walk into a store looking to buy something, but the second a sales person gets in my face and tries to sell me that very item, I walk out. I don't like being advertised to or sold to, and the omnipresence of advertising in sports has turned me away. (this is also why I don't watch TV).
modify - kind of an aside, but at the gym last night I was talking to an NCAA fan who provided the standard arguments for preferring NCAA (NBA players don't try, players are thugs). He was an intelligent guy so we were just kind of debating on it when another fella interjected with the best point around. He knows I am from Boston, and I guess he is from Jersey. He just said it's a cultural thing, mostly. In the Northeast we don't watch as many college sports, whereas in Ohio where I live now, people are bigger college fans than anything else.
I think that's what does it for most NCAA fans vs. Pro fans. They grew up watching NCAA and so that is what they like now.