I think what killed the server was the addition of diamonds. The best feature was the expensive iron block crafting for gear, and then recipes for chainmail. This made it so not every faction had the best gear possible, and it was something you had to work towards. This way you would see players using leather and gold and chain armor, which is cool because you otherwise never see that in factions. So I suggest adding the iron block crafting and chain armor crafting from AoI, and add diamond armor crafted with diamond blocks as an endgame objective.
Also, this way you could reward players who complete quests with Diamond armor, which otherwise would be near impossible to get. PS: Sorry for great text wall of China.
Probably the fact that it was a "small" faction server with no insane cheating. Then it was killed by no other than Challenjour. And then we burned their homes!
Blackfyre and Ilthilien flooded the server with EW people and created more competition, at least that's what I noticed. Once we left, the server seemed to have taken a big dump in activity.
Aaron, basically, it's what szucs said about diam armour being near impossible to get, + all the good plugins they added (gate plugin, crafting recipes for things like saddles, etc). P.S. If you aim to do something like this, then if possible, maybe make horses have higher health, so that those on horseback actually have an advantage in something other than being able to run away faster