After reading about all the traps and the complaining about thiefs and griefers Ive come to a conclusion and that is SMP is pretty much going to be feudal Europe. The only way youll be able to stay safe is with a large group of other people inside some type of fortress or defences. And after the lord took all this time and labor to build said fort why would he just let you freeload inside? Thus you'll have to work in some helpful way for the lord, like a serf. The guards will have to be kept happy by the lord so they dont go and just let an opposing lords army in because he offers better benefits. This could be through giving land or fabulous salaries.
Excuse poor writing for I am writing this while extremely tired.
i'm sure SMP will be like feudal Europe, but underground. No-one is going to build above-ground and risk being seen, so gangs of people will take over huge cave complexes and mine outwards. As soon as the gangs realize that they are occupying the same cave as another group, chaos is sure to ensue. So yeah, it is the feudal system, but underground and on a smaller scale.