What are your criteria for upvoting a post?
As a curie curator I try to find exceptional stuff and curate the best out there, but if there is some good stuff that don't get into "curie zone" I still upvote them too and posts from my feed (known users with whom I interact on steem-chat and discord). Criteria, I really like formatting of the post, how it looks like, the layout - if it is photography post I really like to see some follow up story. If it is long scientific article, I like when it is broken in sections, with some nicely sourced images.

How do you decide how much of an upvote you should give?
Since my upvote worths about 10 cents I throw 100% if I really like the post and if the post is nice but the person is well rewarded I lower the percentage to have more VP for under-rewarded great authors.
Do you leave comments for posts that you did not upvote? Why or why not?
Only if the topic is interesting enough. But mostly not :)
Do you upvote comments that people write for your post? Why or why not?
I upvote comments that are wrote for other posts too. Comments are future of this platform (if they are not begging and spam stuff). Sometimes the comment section persuades me to upvote the whole post too.
What is the one thing that bothers you the most about Steem upvoting?
Too many good posts and authors with so limited voting power. But I guess that is why we have 7 days payout, to recover some VP and cast a vote later. And another thing that will change with the new hardfork selfvoting on first minute, but penalty will come in, and everything will be sorted. :) have nothing against self voting if it is casted after 30mins. :)

Thats all for now :) cheers!