I'm a horrible GM for several reasons.

When you GM you need to make absolutely sure you're committed to it (and that goes for RPing too).  Are you just going through a short 'fad' that will vanish from your mind in a few weeks?  I'm the type of person who sees Transformers, Lord of the Rings, or Star Wars and suddenly want to make an RP based on or inspired by them.  But a week later I realize I kind of don't want to host a RP about them.  This is my biggest weakness.  I get caught up in the hype and then the hype dies down and I'm caught being a GM.  Nothing like joining a fun RP when the host suddenly disappears.  I've done this a dozen times, even here on these boards, and I know it's annoying.

The next thing I'm going to tell you kind of goes hand in hand with commitment.  Make sure you're going to have time to do it.  Gonna work a lot of overtime the next few months?  Going on a cruise?  Going away for a while?  Make sure you at least tell people before you do these things, or wait till they're over and [i]then [/i]host the RP.

Have a clear vision.  You say you already got your world planned out pretty good, and that's a start.  Make sure you know where you're going with the RP, otherwise you'll end up making stuff up on the go and it'll feel convoluted.  If you have a good idea in mind of where the RP should go, specify to the players what type of characters you're seeking.  You will end up getting some very creative, interesting characters, but they won't fit in the RP.  Don't be afraid to tell them they can't join.  Better yet, simply offer them suggestions that might make their character work in the RP.

Like I said, I'm a pretty terrible GM.  But I'm improving.  I've been carrying a RP of mine called 'The Hallows Inn'. since 12/14/2014, which is now 6 months.  That's amazing for me to do.  People are still highly interested in it as well.  But it is on the BattleOn! Forums.