‘Cheapening’ is something you say or do that — without you realizing it — either damages you directly or makes someone else feel like a piece of garbage.  It’s a form of asshattery that’s a lot tougher to manage because, unlike most forms of asshattery, which are overt and intentional acts of disrespect or malign intent, cheapening is an unwitting act of the same nature.  It’s that little thing you do that you either think is entirely innocuous, or that you don’t realize you’re doing at all, that causes very real harm each time you do it. 
You can call it what you want or tweak the definition a bit, but in the end it’s a basic thing that you can encounter in just about every social enviroment.  As a social environment, WoW is subject to it as well.
Despite the unique challenges inherent to the medium, the fact remains that people man the keyboards that generate the words in the various chat channels and people click the mice that make the game sprites do this or that at any given time.  As a result, it would (the operative term, here, I realize) go to follow that some rules of social interaction apply regardless.
‘Don’t Cheapen People’ is one of those rules.  In truth, it has a pretty wide definition that, when broken down, changes depending on environmental factors (such as culture, personality, relationship, type of enviroment, etc.); in short, what makes me feel like dirt may not necessarily make you feel like dirt.  Specifically because cheapening can be defined in so many ways, I want you to focus, instead, on the feelings it invokes  — ’cause those are always universal.  It’s much easier to recognize it when you ask yourself: how would that make me feel?  Those feelings are:
Everybody is human, so everybody knows those.  They taste different to me than they do to you, I’m sure, and maybe you’ve experienced one more fully than I have, but on the whole they’re no mystery — we know what they feel like.  Bottom line is: they hurt.
You don’t want to hurt someone in real life, when they’re sitting in your living room and sharing a drink with you, so why wouldn’t that carry over into the /g — i.e. the living room of your guild?
It can be difficult to realize that you’re cheapening someone.  It’s even harder, ironically, to realize when you’re cheapening yourself.  Fair enough.  I’ll go over a few examples of the phenomenon, anyway, and hopefully they’ll help you recognize the beast for what it is next time it rears its ugly head.
As promised: 
what you did: talk shit about the guild as a whole
who it hurt: you
why it’s not a good idea: generalizing is never smart, because individuals within the group will always wind up proving you wrong.  Let other people form their own opinions, or risk looking like a dick.  And I mean hey: what comes around goes around, so keep that in mind.  
what you did: ignore or disregard a party or raid member’s opinion/input
who it hurt: the other person
why it’s not a good idea: by doing this, you’re telling the other person that they don’t matter.  The very least you should do is acknowledge what the person said — ’cause then they feel they’ve been taken into account/that they’re part of the group.  People want to be included (it’s part of Maslow’s Pyramid, FFS!), so make sure you get in on that.  This is a great way to teach yourself patience and tone (yes, it’s possible even over chat) control, because sometimes it can be really difficult to politely/appropriately acknowledge something that you feel is worthless. 
what you did: spamming the DPS or healing meters only when you’re #1
who it hurt: your audience
why it’s not a good idea: aside from being incredibly annoying, you’re also (potentially) triggering some very negative emotions in some of your fellow DPSers.  I’m not talking about jealousy, so quit pounding your chest.  I’m talking about the darker, stickier emotions that will wind up negatively affecting the morale — in other words, the productivity — of some DPSers.  Not everyone deals with stress or insecurity or what have you in the same way, so if you think you’re making yourself known as a DPS SME by spamming your numbers, think again.  DPSers typically have a DPS meter installed, so they already know what you’re highlighting; if they don’t have a meter, then they don’t care.  If you’re in a raid and you really need someone to step it up, consider rethinking your coaching strategy (we can talk about that in a later post).
Sometimes taking a sec to think before you hit Enter is worth it.  Every second you contribute to that end is a deposit in the emotional bank account of a person (or more!) you play with in-game.  If that sounds too warm and fuzzy, then look at it like this: the more deposits of this sort you make, the less awkward it’ll be between the two of you when you win a shared Need roll V.S. some player you’ve been unintentionally disrespecting for months.