That is my opinion. Initially I was making it a comment, but it quickly became too long for a comment.
Try to use all your votes
People should try harder to hit the daily vote limit and chase the Vox Populi badge. Go on, read the questions and answers and use your upvotes in what you think is good. Do not reserve your upvotes only for the "best of the bests".
Don't forget to vote in questions
I already saw cases of questions with >60 answers but <10 votes. Particularly, if you think that the question was good enough to deserve your answer (and your answer is a serious answer), why not upvote the question?
Instead of thinking in "why should I upvote the question?", think in "why should I not upvote the question?".
Think twice before downvoting
No, do not think only twice, think a third time!
By observing some fenomena occurring here, I conjecture that there are cases in this site of people who does mass-downvoting, since I already saw cases of questions where every answer has at least one downvote and they got the downvotes in a narrow time span. I really can't think how this can be a good thing.
If you look at my profile in the main site, for example, you will see that I very rarely downvote anything. If I think that a particular question or answer is stupid, I will just ignore it or maybe post a comment, and reserve the downvotes only for cases that are really-really deserving them.
On the other hand, I already saw people here that posted more downvotes than upvotes, and I really think that this is just wrong.
Do not be a ninja downvoter
In another community that I participate, this was discussed on meta in the first days of private beta. Somebody post an answer, and in less than a minute it already has a score of -2 (and no, I am not exagerating, in that case it reached -2 votes in 53 seconds), and it was not a bad answer after all. Give time to the person to edit it. Add a comment pointing what do you think that is wrong and give time to the questioner/answerer fix it.
Avoid ad-hominem voting
I already saw cases here of "I never vote on user-xxx answers", or "I always upvote user-xxx answers", or "I always downvote user-xxx answers". This is simply unfair.
Explain your downvotes
If you are willing to downvote, explain why. Getting a random downvote from nowhere without any reason or any explanation is a bit annoying. Getting more than one without a single word about it, is really annoying and frustrating.
Try to observe other people reputation caps
EDIT: [Removed, not practical and possibly counter-productive].
Do not spoil other people fun
I will devote some long text in this because I think that this is the most serious issue here. It is directed specially to closers and I know that a lot of people will disagree.
A question has already some, lets say, 15 answers, and people are posting more new answers. The question and the answers are getting upvotes, people are improving their answers and suggesting improvements in other people answers. But then, it is needed just 5 people to spoil the fun of everybody and close the question only because of a minor problem, or worse, just because of something that the closers think that is a minor problem, but it is a no issue afterall, and then the fun of everybody is spoiled.
I already saw at least 3 cases of a question being closed, reopened and closed again, which make me particularly angry at some people which insists in closing questions for no real issue afterall.
Closing a question which does not have a very serious problem is a disrespect not only to the questioner, but to every people that answered it seriously or are wishing to answer it.
Particularly, already happened 2 times for me that I posted a placeholder answer when the question had 4 close-votes, deleted it, and then undeleted and posted the actual answer after the question was closed, and already saw other people doing the same. I wished that I never needed to do such thing. If people are searching for ways to circumvent the questions closure and post answers anyway, this is a sign that there is something very wrong happening here. And already saw people posting answers as comments with links to github or jsfiddle because the question was closed.
And I already saw cases of questions being closed without a real reason (closing just for the sake of closing), in which the first closer chooses a random close reason in the close menu and other people do follow him sometimes without even reading the question. People closing questions just because they think that they are boring (this is no close reason), or closing question just because it can't be done in their favorite programming language, or even more stupid reasons.
Someone may argue: "We closed because it had a problem and this is to give an opportunity to the OP fix the question". This is red-herring, the OP has the opportunity to fix it without closing. Closing a question punishes much more the answerers than the questioner. And reopening a question is something very hard to do. Further, even if it managed to be ropened, it probably already lost its momentum and is ruined.
Someone may argue: "Even if this is a popularity-contest, it is a duplicate of a two-year old code-golf question which have radically different restrictions". This argument is simply disguting, idiotic, annoying and possibly evil, and sorry this is not a joke, this is real. This only serves to reinforce my feeling that people are just closing for the sake of closing using some lame excuse just because they can do it, and makes me wish that a "downvote the close-reason" button existed. I will not post links to questions where this happened to not expose the names of the closers and because this is off-topic to this actual meta-question.
How to address this issue: Try to be a lawyer for the question, not for its closure. If the question is salvageable, try to save it, preferentially before it gets closed. If it was already closed, try to fix it willing for a reopen. Lets keep closed only what is really unsalvageable and unworthy.
Upvote to counter downvotes
EDIT: [Removed, not a good idea, or at least a very debatable one, and maybe possibly counter-productive].
Do not scare newcomers
Most of newcomers don't know the rules yet, so if a newcomer post a question or a answer that is not in a good format but still clearly shows a visible effort to try to participate, instead of ignoring, downvoting, closing and/or deleting, suggest improvements or, if possible, edit the question/answer to fix any issues. Further, newcomers are much more motivated by a few upvotes than long-established users, so lets motivate them.
Especially, in case of newcomers questions, they are motivated not only by upvotes, but by actual answers too.
Explain why do you close-vote
This could looks like obvious, but unfortunately it is not. If, after reading the long previous section about close-voting, you still thinks that a particular question should be closed, explain why. I already saw at least two questions that already had some answers and were closed without a single word from any of the closers, and it was not closed for an obvious reason (like spam or homework) nor were closed for being a duplicate. Just after that it was closed, some other user who did not close-voted posted a comment like "I think that the rule xxx is a bit underspecified", so ok, fixed this in less than 30 seconds by editing.
Answers attracts more answers, more answers attracts upvotes
If you think that a question is worth, post an answer if you can. Don't need to be that great answer, but this gives you opportunity to get some upvotes for you.
And, even if you are initially not thinking in that great answer, it is possible (uncommon, but possible) that in the middle of the development, you do figure out a smart trick that makes your answer actually a great one.
When other people see your answer, they might get some other ideas for themselves, and think: "Hmm, this guy solved the problem with the X method using the A language, but I think that the Y method is better in the B language, so I can beat his answer. Will upvote him for the idea that he unintentionally gave me."
Reward hard work
It isn't frustrating when you work hard to produce a good answer, and do not get a single upvote on it?
So, please, if you see some answer where the answerer clearly worked hard to produce it and did the best that he/she could, why do not upvote it?
Reconsiders and re-evaluates your downvotes
This was suggested by @DigitalTrauma, I will just post his comment here:
If you do downvote, please check the post later for useful edits and re-evaluate your vote.