[quote]What makes a good GM?[/quote]
Someone with a strong, clearly articulated vision that remains flexible.

[quote]How does one GM a fairly linear plot without making the other RPer(s) feel like you are just pulling them along without a choice?[/quote]
I have a lot of thoughts about this, but I'll just share this one for now: 

I think if you want to tell a linear story, one thing that will help you out a lot is curating what characters you let into the game. If you take just any character from any background, aligning all of those disparate personal trajectories with your story will be pretty much impossible. Tell applicants whatever you need their characters to be before they start crafting. Common enemies, common goals. For a very simple example, all the characters in my game had to belong to a particular mercenary company. To get them started on the same path, all I have to do is have their boss give them orders. Even though in one sense you're "pulling them along without a choice," really it's their characters that chose that life and now they have to deal with the consequences. It makes a huge difference.

Also, to specifically address your point about the story that had crucial events that needed to happen before the main story, don't be afraid to give your the players a prescribed a backstory. Since so many games die because the story hasn't really kicked in, I highly recommend rethinking your story structure, and starting as late in the story as makes sense.

[quote]How can one GM a roleplay and still be involved as a character?[/quote]
I personally choose not to. Maybe it can be done well, but every GM I've seen do this seems to have just figured out the socially acceptable way to god-mod. That said, technically speaking, NPCs are a way to be involved as a character or characters. Just don't get too attached to any of them.

[quote]And any other failures or successes you'd like to share?[/quote]
Here's a failure: there was a point when two of my players were getting along really well and having a lot of back and forth while the other three had nothing to do. So I pushed the story forward, cutting the scene short. What I probably should have done is add something new to the scene for the three players and let the couple keep making magic.