1. On popularity
Even minor obscure Wikipedia articles are read by thousands of people every year; major articles are read by thousands of people every day. This means that regular Wikipedia contributors are some of the most widely read authors on the planet. Anytime you question whether your contributions here have value, remember that you're secretly the J.K. Rowling of the Interwebz.
2. On top articles
You make a bigger contribution to Wikipedia by improving one widely viewed article than a hundred obscure ones. Unfortunately, Wikipedia awards (the WikiCup, user page bragging rights, etc.) tend to encourage the latter over the former, since articles can be created and promoted more easily on minor topics. Forget Hamamelis virginiana and work on tree.
3. On thanking people
Building community is just as important as content-writing, but it's an area far fewer people work on. Track down editors who wrote articles you admire, and tell them so. Thank IPs who make a legitimate contribution, no matter how small, and invite them to stick around. Thank admins for their hard work, thank newbies for their first contribution, and thank even the person you're arguing with right now for the hard work they do and for taking the time to talk to you. The editor who feels valued is ten times more likely to keep going.
4. On policing Wikipedia
As a corollary, few people do more harm than veterans who see policing and correcting other editors as their main job. This isn't to say we should let any mistake slide, but anybody who's ever had a good teacher, boss, or parent knows you can enforce rules and correct mistakes while also being kind and encouraging. (In fact, correcting people usually works better that way.)
Building a negative atmosphere, on the other hand, drives away more contributors (and thus their future contributions) than any other factor. If you see your main role here as correcting or catching out others rather than encouraging or helping them, the odds are you're probably not the vital checkpoint you think you are: you're just a jerk.
5. On dramafests
WP:AN/I, RfC/Us, and WikiProject In The News candidates all have vital functions, but also suck up a huge amount of user energy for little gain. Forums like these burn out a lot of Wikipedians who could have been good content contributors, but turn into arguers instead; they also attract Wikipedians who were more interested in fighting than in content to begin with. Stay clear whenever you can. If you can't, say your piece and get out as if your life depended on it.
6. On laughingstocks
In any sustained argument, the probability that an editor will state that her opponents are making Wikipedia a laughingstock approaches 1.
7. On simple English
Wikipedia's audience includes young and international readers. Use the simplest language appropriate to your subject and avoid idiom. Add a sentence or two of historical context wherever helpful. Don't make assumptions about knowledge: put a "US" in front of "President Barack Obama" and an "England" after Liverpool unless the country in question is already clear from the article.
8. On building for the future
Write for the long-term. Avoid statements that will go out of date in a week or in a year. Cite information in a way that any later editor can easily find it. Archive web citations with a program like Web Citation to avoid future dead links.
9. On tagging articles
Practically nobody cleans up tag backlogs, meaning that most tags are only page-clutter. You can read a great essay about this here. The short version: if you're adding more tags than you're removing, you're not helping Wikipedia; you're just finding ways to make work for more dedicated editors than yourself.