I personally like the ones that emulate the real servers with additional benefits. Healers and warpers, for example. Those are always nice. No level above 99/70, no Naruto headbands, donation items are at best, either mediocre in terms of usefulness, used for showing off, or are consumables which doesn't upset the game.
However, when you get down to the bottom of it, the point of RO is that there's WoE, which is an integral piece that ties the game together. This is why, ideally, you want a constant influx of new players - so that the warring population, for example, are kept in constant check by each other. There is a constant demand for low-tiered gears (mocking mufflers, cranials, elemental lances for leveling, that kind of thing), and also to buffer out the bad feelings that eventually crop up all over the place.
Population and negativity go hand-in-hand. Generally speaking, it's only a matter of time before the nastier sorts end up on any server. EternityRO, for example, has one of the largest populations of any server - I daresay it's got more people than iRO Chaos and Loki at the moment. But the population there is insanely cliqued (which is bound to happen to any game when there isn't a constant influx of newer players, causing the old players to band together in a sort of better-than-you mindset.)
What makes a server great is just that - renewal, whether in population or guild politics or just about everything else. No matter how bad a server is, as long as there are people, the people themselves will try to make it better. I've been on both spectrum, to be honest. On LegendRO I've seen people come and go; guilds rise and fall. At one time, I was even a part of that - someone who tried to make the community a little bit friendlier. That community has now turned to ash. A shadow of its former glory.
But I digress. As stated elsewhere, RO is a dying game - and ultimately, the lack of new players is what's going to kill any server.
I used the examples above because I have no other experiences otherwise. These are only my opinions.