Not very populated, but a very high quality server with a great world and it's also very easy to find a spot away from everyone else if you want to invite some of your friends and just have your own little thing going on.
It has some slight griefing protection, but the playerbase is generally just mature enough to not grief in the first place.
The mods are very laid back. A bit too laid back; when the teleporter in the spawn area breaks, they're usually too lazy to fix it. Just ask around if someone will let you temporarily join their faction so you can teleport to their area and then go exploring on your own.
Sadly I can't say I've seen a server without it's fair share of griefers and annoyances. Best thing you could maybe do is host your own server (or ask a friend to) and use the white list function. Thankfully it's not too hard at all with a vanilla server, nor is it if you would like to use plugins and things (bukkit).