As for the ability to downvote comments, my understanding is that the only requirement is having at least 500 karma.
As of a few years ago I can confirm that. Don't need (as stated by GP) to be active for years, at least, depending on how much and what/where/when you post. Being the first reasonable-sounding or insider-knowledge comment on a new article that then becomes popular gets you ~1/7th of the way with 2 minutes of effort per attempt; being the first to post from an RSS feed of a here-popular blog can get you +500 points in a single go. Just posting regularly, not doing anything special to farm, will get you there in a few months
There is no longer a downvote on submissions.
Do you know the reason? Allowing down voting on comments but not on submissions?
This could be a false memory, but I think it’s the idea that the submission downvotes should be reserved for things that break the rules, and in that case, the post should be flagged instead.