Where to notify us
Logically, just use the "Recent inbox messages" on the top bar.
If it's ok for comments, badges, all other stuffs, then fine/logical also for:
"You got an upvote on Meta, question [link]", or downvote, or comment, etc.
Meta upvote notifications - Meh
I don't so much care about the upvotes.
It matters not because when I post an answer I believe it's a good one and someone will get use from it, otherwise I wouldn't post it.
I therefore don't really need to know about upvotes to have this confirmed, just downvotes so I know it was contradicted!
That said, while I check back in if I have a question on the go, it'd still be useful to not have to manually check back on my question to see if people agreed/disagreed, or did/didn't like the feature-request idea.
Meta downvote notifications - Yeh
Not only would I like to know when someone downvoted my answer or question, but this is required if the site wants people to go back and improve their answer or question, or continue the info flow.
While stack is a Q&A, the Metas are often a little more discussion based, and are never really a hit and run scenario, whether posting an answer or question.
One likes to check back to see others opinion of your question or answer.
For answers, a poor answer is detrimental to the site in general, we should be notified to be able to do something about it - improve it etc.
Perhaps someone downvoted and disagreed. I need to know so I can go debate their response, or agree with them, and either way update my answer to reflect this and thus provide a decent answer to a question for future reference for all other users.
For questions, I obviously need to know, as positive or negative votes tells me if I'm right or wrong, or have a point, or whatever.
Stopping users continuing to participate
It feels a bit, weird, scary in fact, popping back to a meta question and seeing all kinds of activity going on which I was not aware of.
I can currently only avoid this and continue to participate by manually going back there, and this is not practical.
Debates might have skipped several topics in the time I had not checked back, and as such probably too late for me to make comments or suggestions (without loads of comments playing catch up..).
Also, if people are doing this, as they don't get auto notified, then they're wasting time checking their previous Meta answers/questions and thus not spending that time helping out elsewhere.
I think this is something we need rather than want.
Now, how do I know if someone agreed with this or not...I know, I'll leave a tab open and come back now and then :(