Hey there, I found your post interesting, and although I haven't actually done any RP servers, I've been interested and tempted to. According to your questions, here is what I would look for:
When I think of a Minecraft RP server, I think of everything being player run. Jobs, skills, environments, should be done by the players, not plugins. The players should have some sort of physical item currency, removing a lot of command usage that would hurt the experience. Community, would obviously be a huge part; how dedicated would the players be to continue to make the world immersive? You wouldn't want a large part of the roleplay to center around a player that only plans on playing for a day or two, so then you would have to fill in the gaps of the story as to why they're gone.
Simplicity keeps me playing on a server; a lack of superficial barriers. Say if you wanted rules or guidelines, a ruler or leader set them, and if caught breaking them you suffer an in-game punishment, rather than being prevented from doing so in the first place from non-human coding. I personally believe that a really good RP server, would be vanilla.
In terms of the lore, I think it needs to be very natural. Say the server started off as a town, and the lore branched out from that. If players were just dropped into a fully developed kingdom with a long backstory with enemies, allies etc., it feels superficial and hard to get into or be involved. So personally, I would prefer lore nearly entirely made by natural progression. I think rather than assigning a villain (if that's a part of the story you want), someone needs to want to be the villain and have motives.
Seeing the "Old Post" link you provided, it sounds like there is indeed a potential for a good community, but immature "edgy' teens with Hitler skins are unavoidable. If it were up to me, keep it unwhitelisted, and once you feel like you have a good community, make it whitelisted. Additionally, people tend to not want to go through the extra step of asking to be whitelisted, because it's just an inconvenience for a server they may not even enjoy.
For this next section, basically what I originally said about naturally progressed player lore. I think it's up to the players to decide what to do. Admins/mods could leave breadcrumbs or clues to possible story chapters, but forced storytelling isn't effective or fun; a feeling of belonging and community comes naturally.
Lastly I think it's better to have a mix. You'll really never know what will come out of a melting pot of lots of different people, so I think that's what makes online interactions so fun. If you were into a very serious type of roleplay, then yeah, mature players.
