I was a good, proud guild leader of a 100-man guild. We defeated other guilds in guild wars (we were undefeated!!!). We held massive in-guild RP events. I remember how we had a ranking system based on how many enemies you killed/resources you brought into the guild. The highest rank of knight involved an elaborate ceremony at the Bronze Doors on the tip of the eagle head, where we would "knight" the player using /me commands and then give the player a ceremonial sword. It was awesome.
Then a couple major updates came out, causing most of our members to quit, and now the guild is disbanded.
Hurrah.
Aside from that, I would say a good guild leader needs to keep the guild interested, needs to keep people busy trying to up their ranks and also needs to host some guild events. A guild war makes people really like to be in the guild (unless you pick a fight with Klyros Junction). At least, that's how I remember it. Oh yeah, and be online a lot. Nobody likes it when there isn't always a high-ranking official to give you points.