Hmm, not gonna bother with my ideal classic server since I've attempted that. I'll go with RPG.
A little while ago I was going to make a small RPG "demo server." It wasn't going to much, but it was going to be fully functional. The whole reason for this was to create the base of what I thought my ideal RPG server would be, even if it wasn't an actual full server. I had some basic ideas laid out, but in the end it all came down to putting actual effort into what I was doing. It seems nearly all the servers out there, no matter the genre, put as much effort to get what they want working and leave it at that. Where's the final touch? Where's the details?
First, I wanted to create a responsive combat system. This meant first creating mobs that were smooth clientside, yet secure and synchronized with the serverside. Then there was accurate hit-detection. I wanted players to feel the energy being put into a sword swing and see the response immediately. When you swung your sword all monsters in range would fly back, damage numbers would fly out and bounce around and then you'd finish them off with a nice combo. Why? Because **** like that looks awesome and that's what players want to see. They don't want to have to align their player so their single-pixel hit detection actually hits the monster jumping around the level because it's serverside, even though they have a sword 3 times bigger than their own sprite. Then, finally when making the kill all the experience bursts from the monsters corpse and gets absorbed by your experience bar!
Anywho... then it's just about variety and giving the players options. Why only limit all the players to bash on the same level 5 monsters? Create a large expansive world so players can explore and do what they want. To be honest, in WoW even though there were tons of the same retextured mobs, and tons of say, monsters all at level 5 I felt like I have options to choose from. Didn't want to kill wolves? Fine, I'll go over to this other part of the world and kill bandits. There were TONS of different places to accomplish the same goal, and I think this should be the DEVELOPERS goal. Even if the player ends up grinding, at least they don't feel like it's a grind, ya know?
On top of that, players like to show off. I think a huge key to an RPG server is to give players the chance to obtain things other players can't, or don't have the patience to obtain. In the end it makes them feel more accomplished even though what they did really wasn't much of an accomplishment at all, just a lot of grinding. Then they can go around and show off other players how cool they are and inspire them to want to be different too. However, if you do this you need to make sure there is enough out so all players aren't just looking the same.
Then it comes down to actual setting. Make the world beautiful and a joy to explore. You don't know how important this is. Players LOVE to come across some abandoned ruins to find out that it's infested with ghouls only to then come across a journal explaining the inhabitants demise and that they ARE the ghouls. Stuff like that is awesome. Backstory is a great way to keep players interested on the side. Exploring the world makes going from one grind to the next not so bad.
So... this was more of some mindless babble than anything, wasn't really organized. Don't forget the standard stuff like guilds and parties and all. In the end it just comes down to effort and attention to detail, and of course skill. No offense, but no matter how much effort you try to script and make graphics if you don't have the skill then it's not gonna be appealing. That doesn't mean you CAN'T have the skill, but that just takes more effort.