Playable player count, responsible staff team that always has people online, and HCRP ruleset.
Also, the map the server uses needs to be designed for lockpickers, theiving is my favorite playstyle. What I mean is, don't just have chests. They should be placed so that people downstairs in a castle can't hear people upstairs picking in. Use teleport doors or something to make the chest room be far away. They only map I've seen that did this, was my favorite map because of this. The only people that could detect lockpickers should be the people standing right behind them, not the people in the same building. Less stupid, more fun.
Another important thing is hunting, and realistic looking terrain. I'm talking like archipelago of yore, with its dense forests that feel real. I haven't seen other maps that have gone as far as Yore has in this direction. It looks like it should be very lagy, but somehow lore did it in a way that isn't too memory intensive.
HCRP servers should have maps with decent castles. All castles need a dungeon, a throne room, and bedroom areas. Bonus points for a dining hall, training yard, etc. The more areas for RP, the better.
The server would also have a village faction that is uncapable, and a monastery designed to always be at peace, also uncapable. I didn't like how on Nordheim map, the monastery faction sometimes got taken over by unpeaceful people and it went on "crusades". It ruined the RP for the peaceful monk RPers.
Another thing is resources. They all have to be scaterred about. Linen should either be cheaper than it is in vanilla, or flax plants should be placed in the woods, forcing people to venture out to get them. Therer should only be one large iron mine, which is in control of by a capable faction, encouraging people to fight for it. Resource wars are something I haven't seen much in PW, but is the best type of war IMO.
EDIT: and this is pretty much my edited arch of yore: Golden Age of Yore.