Taking charge. Too many people are guilty, including myself, of being a passive GM. You need to be upfront and assertive. Don't accept a terrible sheet just because you accepted everyone else. If someone is being a little shit and bothering the rest of the players, kick them out of the RP instead of twiddling your thumbs and wondering why everyone is leaving. Try your hardest to stay focused on the RP i.e. post on set deadlines (this one I'm especially guilty of not doing) and being an active role in the RP (albeit without going overboard; let the players have some reign). Etc.
Kuro did manage to nail some of the failings that players tend to see in bad GMs, but it goes a lot deeper than that. GMs need to know how to make interest checks that will be interesting for their target audience to read. They need to be good with working with a lot of different personality types, need to make all the players feel welcome in their RP, while also knowing what to do if one of them suddenly drops. They need to be resourcefull and know what to do when one of their story hooks won't work. They need to be able to plan ahead so that when shit goes south, things continue to move forward. Beating your chest and telling people "no" is one of the easier things to do.
If you want a comprehensive write up of how great GMs operate, a sexual icon known as @Dervish wrote a great guide on the topic which you can find here.
It should be noted that like anything, you can only absorb so much from reading. You do have to learn from your mistakes and get a feel for being a good GM. I've been doing this for a very long time and I'm still learning how to do things better.
To answer the second question, the most successful GMs have perseverance. Anyone can have a one hit wonder, but it's the ability to pull yourself together after a disastrous flop that lets you go on to make more RPs, hopefully even better than the last.
Follow up question, what do you think makes for a most successful GM?
Patience, Luck, and Chemistry.
But mostly luck.
To expand on this, GMing is just the science of circumstance, timing, and planning. Planning too much dooms yourself to fail and expecting everything to go to plan will never ever happen. You’ve got to have a group you have chemistry with and a mutual sense of patience. Maintaining interest isn’t determined by constant activity (though it does help) but rather the will of the community. As a GM you are a director that needs to have a finger on the pulse, but even if you have that down… it’s mostly luck and circumstances aligning.
Knowledge and understanding, too. I think it's incredibly important for a GM to know WHAT they want and WHO they want to participate in an RP. Furthermore, they should know WHERE they want the RP to go. That way, there's cohesion and direction.
As someone currently running an RP, I would say making sure to keep a consistent sense of structure and framework to the setting while facilitating an environment that encourages player interactivity.
In the context for a forum NRP, that to me means making sure that anything the GM does is in the name of acting as effectively a prompt generator and as the one who steers the general direction of an RP.
When running an NRP, my tendency is to usually make factions that serve a story arc purpose other players can use as a foil for their own societies. This method can work to pretty great effect by giving some central antagonizing force to handle or otherwise something to reference from.