If you aren't sure why "thoughts on kindness" are in a thing about playing, please bear with me for a bit. This may be sort of obvious to you, or it may be crazy stuff you've never heard before. I know that a lot of people have been told stuff about "kindness" that makes them hate it or think it's stupid.
Kindness is hard for people to comprehend. Kindness does not mean being a doormat. It does not mean always yielding dominance in social status disputes, it does not mean giving people things you aren't willing to give them.
Being genuinely kind is a great way to make your world more pleasant. People appreciate it, and it makes them happier and friendlier. Sometimes, you're going to really be focused on it making them friendlier, when you need help or something. Sometimes, you're going to be more focused on it making them happier, when you're doing fine and you want to share the good life. Either way, being kind is a good skill, and one that most people are not only not-taught, but are sort of anti-taught by schools and parents and such.
This is a bit of a debate topic. So I'm not going to claim that I am talking about the absolutely universal sense of the English word "kindness". Rather, I'm using one of the many senses of the word, but this is what I'm talking about.
What I'm really talking about is the thing that is sometimes referred to as "love" or (in older English) "charity". You know, "love is patient, love is kind", that thing.
The thing here is... This is mostly a matter of attitude; actions can come from this attitude, but the attitude itself is what changes your experience. The magic doesn't come from what you say or do, but how you think about what you are saying and doing.
So here it is in a nutshell: Like people. Want them to be well and happy. Note that "happy" can be a little indirect sometimes; if someone's got a booze problem, not-drinking might make them "unhappy" in the short term, but might make them a lot happier in the long term. That's why I say "well and happy"; you want people to be fulfilled, not merely satisfied.
And now, with the "what I mean when I say this" out of the way...
Don't interact with roles. There is no "cashier" or "clerk" or "officer", there is a person who happens to be doing a thing. Being treated entirely in terms of your role is, unsurprisingly, dehumanizing. It drains the life out of people. Being treated as a person who happens to be doing a thing brings that life back.
Same thing in MMOs. Don't think "healer". Think "player who is currently running a character who is healing". You might yell at a "healer" for being "bad", but would you yell at a person for not playing a game well enough? Well, maybe you would, but you probably shouldn't. Thing is, it's a lot easier to remember that if you're thinking about the person, not the role.
Try this experiment: When people in service industry jobs ask you how you're doing, give it a moment's thought and a true answer. Say things like "I'm having a great day!" or "Honestly, I'm having a crappy day, which is why I'm taking a break for lunch." Watch what happens. And then, ask them how they're doing; you may be surprised to find that they actually have lives and experiences of their own!
Dale Carnegie's classic, How to Win Friends and Influence People, is full of detailed accounts of how insanely effective this is as a tactic for improving your world.
For some reason, people tend to think that kindness is all about giving up everything you want so other people can be happy. Not so! In fact, in many cases, being a doormat for people is not actually helping them at all.
Again, you want people to be well and happy. A state of arrogance or dominance is not a healthy one for humans; it leads to unhappiness. So you don't want that, and you don't want to encourage that.
Basically, if people think they are entitled to something, giving it to them is not kindness. Either they really are entitled to it, and it's no special kindness to comply with this obligation, or they aren't, and giving it to them encourages them to think wrongly about the world.
So be clear on your boundaries. Don't go around assuming other people matter more than you do, don't give up things you desperately want because they're slightly inconvenient to people. Don't immediately back down whenever anyone challenges you, or tries to tell you how to act. Doing these things is bad for you and bad for them.
See, all that stuff about "being nice"... it actually can work, once you've got those previous two components in place. If you are aware of someone as a person, and you are not being a doormat, you will find that there are many ways to be nice to people... and that they actually have some effect. People often think that "kindness" is just a strategy for losing, but they're wrong. The problem isn't really that "being nice" doesn't work; it's that it only works when you approach it in terms of actual people and reasonable understanding of boundaries.
Don't be afraid to do small things that don't seem like they'll have a huge impact. Common courtesy is huge, and people don't do it much. Take the common issue of, say, people who send messages in an MMO asking whether you're interested in something or not. A polite "No, but thank you for asking" or "I'm really not up for socials tonight, but I appreciate the offer" can be transformative. Why? Because for most people, even social ones, approaching strangers is always a bit of an emotional risk, and having it just go completely ignored can make them feel awful. Getting a friendly response, even a "no", can help a lot. Similarly, answering trivial questions from newbies makes a huge difference in their experience of the game.
Simple stuff can be huge. One of the best days of my entire life was a horrible day I was having, and I was absolutely at the wall, I could not handle ONE more thing, and it was trash night and that meant we had to shovel the driveway. Could not face this. ... And then I looked, and our neighbor had gone over the driveway with his snowblower. Seriously. This is one of the best experiences I have ever had. Words cannot describe how absolutely wonderful that was. It made a horrible day wonderful.
Remembering music someone likes, remembering how they like their burgers, stuff like this makes a huge difference to people because it lets them know that you are aware of them as a person, not just as an event happening in your life.