I've been tagged! Man, I totally missed this one! D=
I haven't been playing MMOs for 5 years yet, but I think I've got 5 lessions anyway.
1. A Guild Lives or Dies By Its Structure
Organization is the key to success. Write down your guild's values and goals, and uphold them. Support your teammates, think about morale. This is not just for raiding guilds; any strong community needs open communication, structure and goals to thrive and grow.
2. A MMO is Only as Fun as The People You Play With
In a MMO, the people you're around make a big difference in the atmosphere of your experience. Being a guildless soloer who only does pick-up groups gives you a totally different feel than a person in an active, close-knit guild. I never would have stuck with WoW past the first year if I hadn't had my guild.
3. Your Gaming Time is Your Own
Running lowbies through content, giving gold, etc. is fine... but be aware that some folks will milk you for all it's worth unless you put boundaries on your time. It's not a bad thing to log in and just want to play for yourself, don't be ashamed to stand up for that.
4. I Love You IRL, But Can't Stand You Ingame.
It's not always ducks and bunnies to get RL friends/significant others playing the same MMO as you. =P
5. Remember to Have Fun
Sometimes, you just need to sit back, stop the rep farming and instance grinds, and take a ride thru the zone you went through at level 2 and wave at the lowbies. Find creative ways to do silly things. Remember to make your own fun sometimes, and don't get stuck on the treadmill.