One way servers fail is due to when pple start drifting off and they drift off because they are either veteran solo players or group players and the server becomes too easy or too hard for their gameplay type. This factor usually surfaces with server population at higher levels.
Those who like to hunt in groups don't like to see bosses being soloed or hunting caves where you don't need to hunt in groups, when grouping up with others in fact gets into the way of leveling (either too few mobs to be had, or mobs too easy for group effort etc) and group play oriented pple start leaving.
And solo players get put off by excessive hardness when you can't solo bosses or even hunt in some caves effectively due to them being made hard to suit group effort...
I'd like to see servers defined not only by 'rate' but also by solo or group playing orientation. On solo server grouping up function wouldn't be there, or if it was you would get such a punitive exp cuts the bigger the group was that nobody would group up for leveling purpose. And KRs woudn't allow groups in them at all. You could still join with your buddies but only one of you would get exp for kill, only one could pick drop before it would open to all, only one would get quest point if the boss was quest mob...
And vice versa on group oriented servers - solo you wouldn't be able to enter KRs, subs would be made hard to discourage solo players, end game items would be open to get only to group efforts, solo players might buy those from TM only...