When I google something and I find the answer on Stack Overflow, I always upvote the question and any useful answers. I can't do this on Super User or any other SE site, because I don't ask or answer questions on those sites. Why? I understand the anti-spam reason for the 15 rep minimum, but why do you apply that same minimum value to users that are demonstrably active on another SE site? All it does is preventing me from adding value by upvoting useful content.
What is meta?
- Any Stack Exchange user can ask a question
- The community provides support, votes on ideas, and reports bugs
- Your voice helps shape the way Stack Exchange operates
This is already in place, but to be "demonstrably active" the system requires 200 reputation, not 100 as you suggest. Once you reach 200 on any site, you will get a bonus of 100 on all your linked accounts (including the one that reached 200).
I was thinking about this yesterday. But answer is simple. If you receive > 200 reputation on SO, you'll get 100 rep bonus on each site you're & you'll connect with, and then you are able to upvote.