Well, I for one think that the MOST important thing any experienced hobbyist can do is encourage the growth of our hobby. I started with a 10g tank back when 10, 15, and 20g steel framed tanks were the standard in the hobby, I started with tetras and such. This seems to be the way most people start out even today.By giving patient, clear, and encouraging responses to such newbies we potentially will increase the diversity and availability of fishes and aquarium products in the future.
Help our hobby grow.
Help the new ones to understand the responsibilities and ethhics involved in the hobby.
Remember that you were new once too.
Oh, one other thing,
don't be afraid to back down from a position that you have taken, sometimes you can be wrong no matter how experienced you are. Sometimes this is do to misunderstood or misremembered info, sometimes it is do to advancements in the hobby (such as that early keepers were encouraged to never disturb the substrate, or the fact that no one new that cory cats were shoaling fish).
Early on in my membership here I made a statement that pike cichlids often hybridized, this was do to misreadinga couple of unrelated articles and listening to someone who actually did not know what they were talking about. When informed that I was wrong I checked my sources and found out that I was indeed wrong. I apologised, updated my ideas on the subject, and moved on.
Remeber, ignorance can be corrected.
Guppy