[Wouldn't it make sense to have Premium Mature Sandboxes?]

https://jira.secondlife.com  is where you make suggestions.

[Who is the target audience for Second life?]

Who was Second Life created for in the first place, and has that target audience changed through the years?  I don't feel like I'm in the target audience.

[Any avatars you're not using?]

Who says names have to have numbers?

[small in real life but not allowed in SL?]

I know someone who has built a few motorcycles in SL, and to get everything to look right when compared to avatars he multiplied the RL dimension by 1.25.  Take your RL height, etc. and multiply by 1.25 and se how your avatar is perceived by others.

[Registration sites that still use last names?]

I have often wondered why new users make up stupid alpha-numeric names.  Just make up a first and last name and forget about the space between the two.  Newer viewers don't show the "Resident" part so all is cool.  What bugs me is Chat showing Display Names along with Legacy Names.  Since I don't use Display Names, i respond to the legacy name in Chat.