Personally I think there many different thins that can make a great GM but that a great GM doesn't need all of them, for example a GM that is great with high fantasy rp's may be mediocre when it comes to super heroes rpgs as they require a different plot. And than there is of course the difference between free and casual or even more extreme table top vs "standard" rp's. As for your specific questions I will try to help but I have to warn you the few times I GM'ed I was terrible, since knowing how to do or thinking you know is in no way the same as actually doing it.


[b]■What are great ways to keep players engaged both IC and OOC?[/b]
 
IC is pretty simple in theory make an interesting plot and make sure that every playing character has significant influence in how it develops. The last freee rpI was in eventually became the GM and another person posting back and forth thus making the other players little more than npc's unable to really affect the story. 

As for the OOC you can allow them to guess plotpoints and even letting them know that if you like their idea's enough you will actually use it, but the main thing is I think that the Gm themselves has to be invested in the OOC.

[b]■Should the character submission be the only factor in accepting players?[/b]

I myself don't think so, of course it should be the mainfactor but someone who you know is a good and interesting rp're who won't leave quickly can take priority over someone else even if the quality of the CS'es were close. You should  make it clear when you do this though, preferably in the Interest Check so that everyone involved will know about it.

[b]■How do you deal with the players who have seemingly disappeared from the RP? [/b]

A simple solution is to put up a timelimit and kill their characters off when it's expired and allowing other layers to take their places or if they prefer to take over their characters, once again you should let this be known along with the timelimit and maybe allow possible exceptions due to RL circumstances if you and the other players are informed.   

[b]■In the same vein, how do you effectively prevent the above from hindering your other players?[/b]
See the above

[b]■What is the most graceful way to end your RP due to inactivity or (*gasp!*) your own loss of interest/time?[/b]
No idea