The what's hot section is just as lazy and useless as Twitter trending topics. I almost never find globally hot topics hot. Twitter occasionally has some semi-relevant content there, but Google+ hot topics are dominated by funny pictures and gifs. I can only see this problem growing further as a result of these changes.

This seems like a problem that is just asking to be solved. I've thought of this before: Just like you can use Location to affect what topics are hot for you, if they could determine a way to select area of interest to filter them it might prove useful once again.

I'm sure it isn't a trivial thing to solve; but not impossible.

Google may already have determined your interests via their Ad profiling. They got me pretty spot-on, you can see what they've pinned you down as liking at http://www.google.com/ads/preferences

However, lots of teenagers will be glad to be on google+, I've felt extremely frustrated at not being on it not because it's a "hip" social site, but because of google's constant integration of google+ into everything. It was like going to disney-land without seeing Mickey Mouse, so I hardly think the complaints would be justified.

Also, not to forget lots of teenagers are tech-oriented, let's not forget how apple and Microsoft started!