So any lessons YOU had to learn the hard way? I'll go first

Don't go AFK in Toronda...aggro crabs FTL
Don't greet Gorgulu repeteadely
Don't even greet Ashtorath ONCE!
And never...and I mean EVER smile at Luciphage.

So, what about you guys? And girls...and maybe possibly Divine people

Don't ever blame Terentia for anything. Also, never remark to her about any unpleasant experiences on which you have missed out.

Don't ever challenge Morgfyre to a debate (actually I learned this by seeing someone else who learned the hard way).

Female dwarves exist only in legends.

Never space out unless you are in your guildhall.
Never leave your hunting companion outdoors where someone can kill it.
You can indeed make your commanding officer feel bad for leaving you alone on Nil by accident.
Lady Terentia has a warped sense of humor.

Never depend on a borrowed cubix for a quick getaway when you're low on health potions.
Terentia likes crispy followers.
Don't use the pants emote on giant clams.
Using glamours to impersonate star council members is frowned upon.
I can't kill Gorgulu alone, even if he is shielding.
Malicia gets mad when you target "mal" while fighting Malarious.
Don't go to 3v3 fights where you're the only non-demigod and thus the softest target.
Being a bard in group combat gets you abused a lot.
Don't bash with Forren.

All I can think of for now

Qft.

Fixed up for you two!

Explainplz?

Divine mindset or some such, makes the Divine respond with the best debate answer regardless of what you use.

Boring! I was hoping he meant a REAL debate.

I actually think he means when someone started debating Morgfyre, and he was flung to astral.

Don't play slap a divine and run. They don't get amused.

I think Morgy sent Malarious to Astral for debating him.

Never irritate a god, even the slightest. The moment they have an excuse to DF you, they'll use that as an excuse to screw you over as hard as they can, and as long as they can, as well as citing "various other incidents" that may or may not have ever happened.

If you're alone in an aethership, don't fly near the vortexes west of Celest.
Flying into one mine at a time is a good idea, flying straight into a minefield is a bad idea.
In wargames, always choose the team with the most monks.
Having the most powerful demigod/titan/combatant on your side doesn't always mean automatic win, you need to fight too.
When in doubt, tumble.
When in choke, tumble.
Don't bash with Forren.

Things I have learned in my short time here:

That unexplained shadow emote wasn't just an "odd random outdoorsy" message.
That odd pile of sand is not covering valuable treasure.
Do you look cool as a Serenguard in black leathers carrying a katana? Yes! Are said leathers a prudent choice for a Serenguard when fighting anything more threatening than a weevil? ...not... so... much. No.

One of my first lessons, actually. I remember exploring the blasted lands under innocence and thought there was treasure. I just happened to get a shovel after I got rid of innocence and went on a treasure hunt.

I wonder if I can sell maps to the blasted lands along with shovels and claim there's treasure out there, buried under loose piles of sand.

Don't blow 50k+ essence regularely on greeting random Serens.. randomly.

Don't bash with Forren as faeling if you're not up for a thrill.

Don't tell Terentia she's responsible for your death if you don't want to get crispy.

Don't challenge Charune to behead you.

Don't stick around Tael for any amount of time. He'll shed on you.

Don't let Nejii into the havens unobserved. He sheds too.

Quit using 'Lis' to refer to Lisarel >_<

Eh, Forren's not so bad. He dies more often than I do when we go bashing.

Most of the ones already stated can apply to me, but I'll add one more:

*Simply because every denizen you've run into in the past is complacent to watch you beat up his buddies and wait his turn doesn't mean all the others in the Basin will grant you the same courtesy

Playing an unaggressive character does not deter aggressive denziens.
Do not fly in the mountains.
Allheale does not cure everything.

Do not be afraid to ask for help. Just because you're embarrassed that you ran into a Glomdoring totem and are hallucinating and proclaiming your love of cheese does not mean you should log out and wait until the majority of the playerbase is sleeping before you find the herbs to cure yourself. *cough* I mean. You're missing out on a chance to roleplay and interact. *sagenod*

Glance when you're in the Blasted Lands, for the love of God.
Satiation is more cost effective than buying twice the amount of food.
Don't guildhop. You're only going to end up where you started... sans 2000 credits or so.
"Rape" is the customary alias of users of ColourMaelstrom, Choke and PPK. (Less of a lesson, more of a random fact)
Don't roleplay a lunatic if you can't handle the crippling, mind-bending consequences.
"Shoot first, ask questions later" totally doesn't work here.