Siege is unique. I've never found a production shard with such a strong sense of community, for starters. Occasionally, there will be a really tight-knit guild or two, but for community, Siege wins, hands down.
The ruleset's also great. Moan about PvP all you like - sure, it's unbalanced, sure, the lowest common denominator will always head for the template or items that give the easy win - so what? I still enjoy it.
But, most of the time, you can walk into a gathering on Siege, as a complete stranger, and by the end of the evening, feel like you're practically part of the family. (alternately, you can walk into a group of PKs and end up as one of their slaves, but that's fun too!)
I've tried most of the other shards that I have a reasonable ping to, and I've tried Mugen, Siege's 'sister' shard - and there's nothing like this place.
Now, if they'd just board up Luna for a weekend or two, it'd be perfect!