In order of frequency:

-I strong downvote spam (weekly)

-I downvote people for antisocial behaviour, like name calling (monthly)

-I sometimes downvote comments that are obviously unhelpful or wrong (I'll usually explain why, if no one else has) (every couple of months)

-I occasionally downvote posts if I don't think they're the type of thing that should be on the Forum (for example, they're very poorly written, very incorrect, or offensive) (a couple times a year)

My general algorithm for voting is to vote up that which I would have liked to have recommend for me to read and downvote that which I would be disappointed if it were recommended to me, where the criterion for wanting something recommended is does it thoughtfully engage with a topic in a way that advances my understanding (and in the case that my understanding already includes what is presented, I try to imagine the case that I didn't know what I know and vote from that place of counterfactual ignorance). I don't vote on things that either fail to pique my interest or that I feel indifferent on having recommended to me.

Strong votes (up and down) go to things that I would, respectively, be visibly happy or sad if someone recommended it to me, i.e. someone sent me an email about it and I light up and smile or frown and droop when I read the content.

Most often I downvote posts when I'm reasonably confident that it would be a waste of time for others to open and read it (confused posts, off-topic, rambling, trivial, etc.)—my goal with voting is to make recommendations to others.

I rarely downvote comments, typically only when someone's not playing nice, but that's more on LW than here.

I rarely downvote EA Forum content.

When I do downvote, I downvote spam & bad-faith rhetorical moves (like ad hominem arguments).

I very occasionally downvote good-faith arguments that appear to have been constructed sloppily or in a rush, such that they don't address the arguments of the other interlocutor(s).

I share some of the other reasons put forth by other answers. But the best single factor I can use to explain the output of my system is the question I often ask myself: "Is this helpful?"

("Helpful" = giving people information they can use, improving the overall epistemic condition of the EA community, providing more utility than they require time to read/understand, making it more likely that good conversations will happen on the Forum in the future, etc.)

Spam is unhelpful. Insults are unhelpful. Sarcasm and snark are usually unhelpful and almost never actually helpful, with rare exceptions where they help someone get an important point across. Low-effort posts and comments that don't add substantive information are neither helpful nor unhelpful, so I generally don't vote on them.

Whether I disagree with an opinion or not has no bearing on whether it is helpful; I have to pay attention to other features of the opinion, like how well it uses sources to back up its claims or how clearly it is expressed.

I downvote posts when I disagree with them, they rely on an argument that is obviously faulty to me, and I think the current score is too high. I feel much freer to downvote if the current score is higher, and often ignore the second condition if it is particularly high.

I like having the vast majority of posts have a positive score, with only spam or name calling having a negative score, as it is on the EA Forum, but I don't really see an issue with yay/boo voting. It's hard not to interpret scores as a combination of agreement and good argumentation.