Throwing my thoughts on here.
Basically people who are saying that osu will be filled with too many kids.. don't even remember that around 60-70% of the pros aren't even of 25 and older.
Even the top players are students. Therefore this statement is invalid. The thing is "immaturity" which I think pretty much most large communities will have some. I won't call osu a huge community, but since this game is pretty competitive and the pros are pretty unhuman, it is not surprising to see people spamming phrases like "wtf", "not even fc" in chats. It's just natural for a game that difficult. (Unlike games like Starcraft, where everything depends on skill and strategy, this is basically a game of talent.)
And as for copyright, I recall that a feature might get implemented in the future, that users have to import their own mp3's in order to play a beatmap, and only files such as .osu(beatmap file), .osb(storyboard), hitsounds etc will be available for download. I personally think it is quite good in this way, as people will have to get a legitimate copy of a song (or even crop songs out of videos, whatever), and therefore avoid issues with copyright if osu is to become bigger, since the songs aren't in the downloads. I don't know if this will ever be implemented, but I think it's a good idea that should be added.
I see that osu streams are mentioned as well - as much as I want to argue that osu streams can be exciting to watch when paired with a handcam (possibly?) I have to agree it doesn't really appeal to random stream passer-bys. Probably the best way to advertise is still through videos and word of mouth.
I really hope for osu to grow big though.
But I think the osu team needs to get bigger though, to do things like fulfilling some popular requests, such as graveyard maps' leaderboard, and make it a good way to show outsiders that apart from user-generated content, the creators will offer new content based on the community's wishes, and attract more people to join the game for the awesome staff the game has.