Personally I think that the casting of TNG was spot on perfect. Few of them may have been well known prior but they fit their characters like a glove, and you coul tell that from the minute they were onscreen. You could see it, and when that's the case their likeability and charm saw you through below par episodes.
I totally disagree they weren't a family, they were shipmates to start with, but I defy anyone to watch the last scene of 'All Good Things' and not see they had all become more than that.
It also benefitted from better effects and bigger budgets (most of the time, but it was the 1980s!)
It helps in my affection that TNG was my first Trek series, so naturally I will have a closer affinity for it, but I also agree with all the other positive comments posted by others about the show. I think DS9 is the best overall series, thematically and emotionally, but I don't love it the way I love TNG.